<XBRL>
<?xml version="1.0" encoding="US-ASCII"?>
<!--XBRL Document Created with Wdesk from Workiva-->
<!-- -->
<linkbase xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" xmlns="http://www.xbrl.org/2003/linkbase" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <roleRef roleURI="http://www.labcorp.com/role/BasisOfFinancialStatementPresentation" xlink:href="lh-20160930.xsd#BasisOfFinancialStatementPresentation" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/BasisOfFinancialStatementPresentationDetails" xlink:href="lh-20160930.xsd#BasisOfFinancialStatementPresentationDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/BusinessAcquisitions" xlink:href="lh-20160930.xsd#BusinessAcquisitions" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/BusinessAcquisitionsBusinessAcquisitionsInAggregateDetails" xlink:href="lh-20160930.xsd#BusinessAcquisitionsBusinessAcquisitionsInAggregateDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/BusinessAcquisitionsBusinessAcquisitionsTablesTables" xlink:href="lh-20160930.xsd#BusinessAcquisitionsBusinessAcquisitionsTablesTables" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/BusinessAcquisitionsDetails" xlink:href="lh-20160930.xsd#BusinessAcquisitionsDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/BusinessSegmentInformationBusinessSegmentInformationDetails" xlink:href="lh-20160930.xsd#BusinessSegmentInformationBusinessSegmentInformationDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/BusinessSegmentInformationBusinessSegmentInformationNotes" xlink:href="lh-20160930.xsd#BusinessSegmentInformationBusinessSegmentInformationNotes" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/BusinessSegmentInformationBusinessSegmentInformationSegmentReconciliationOfOperatingIncomeToConsolidatedTables" xlink:href="lh-20160930.xsd#BusinessSegmentInformationBusinessSegmentInformationSegmentReconciliationOfOperatingIncomeToConsolidatedTables" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/CommitmentsAndContingencies" xlink:href="lh-20160930.xsd#CommitmentsAndContingencies" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/CommitmentsAndContingenciesDetails" xlink:href="lh-20160930.xsd#CommitmentsAndContingenciesDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/CondensedConsolidatedBalanceSheetsUnaudited" xlink:href="lh-20160930.xsd#CondensedConsolidatedBalanceSheetsUnaudited" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/CondensedConsolidatedBalanceSheetsUnauditedParentheticals" xlink:href="lh-20160930.xsd#CondensedConsolidatedBalanceSheetsUnauditedParentheticals" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/CondensedConsolidatedStatementsOfCashFlowsUnaudited" xlink:href="lh-20160930.xsd#CondensedConsolidatedStatementsOfCashFlowsUnaudited" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/CondensedConsolidatedStatementsOfChangesInShareholdersEquityUnaudited" xlink:href="lh-20160930.xsd#CondensedConsolidatedStatementsOfChangesInShareholdersEquityUnaudited" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/CondensedConsolidatedStatementsOfComprehensiveEarningsUnaudited" xlink:href="lh-20160930.xsd#CondensedConsolidatedStatementsOfComprehensiveEarningsUnaudited" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/CondensedConsolidatedStatementsOfOperationsUnaudited" xlink:href="lh-20160930.xsd#CondensedConsolidatedStatementsOfOperationsUnaudited" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/Debt" xlink:href="lh-20160930.xsd#Debt" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/DebtConvertibleSubordinatedNotesDetails" xlink:href="lh-20160930.xsd#DebtConvertibleSubordinatedNotesDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/DebtCreditFacilitiesDetails" xlink:href="lh-20160930.xsd#DebtCreditFacilitiesDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/DebtLongTermDebtTableDetails" xlink:href="lh-20160930.xsd#DebtLongTermDebtTableDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/DebtSeniorNotesDetails" xlink:href="lh-20160930.xsd#DebtSeniorNotesDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/DebtShortTermBorrowingsAndCurrentPortionOfLongTermDebtTableDetails" xlink:href="lh-20160930.xsd#DebtShortTermBorrowingsAndCurrentPortionOfLongTermDebtTableDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/DebtTables" xlink:href="lh-20160930.xsd#DebtTables" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/DerivativeInstrumentsAndHedgingActivities" xlink:href="lh-20160930.xsd#DerivativeInstrumentsAndHedgingActivities" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/DerivativeInstrumentsAndHedgingActivitiesDetails" xlink:href="lh-20160930.xsd#DerivativeInstrumentsAndHedgingActivitiesDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/DerivativeInstrumentsAndHedgingActivitiesTables" xlink:href="lh-20160930.xsd#DerivativeInstrumentsAndHedgingActivitiesTables" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/DocumentAndEntityInformationDocument" xlink:href="lh-20160930.xsd#DocumentAndEntityInformationDocument" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/EarningsPerShare" xlink:href="lh-20160930.xsd#EarningsPerShare" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/EarningsPerSharePotentialCommonSharesNotIncludedInComputationOfDilutedEarningsPerShareDetails" xlink:href="lh-20160930.xsd#EarningsPerSharePotentialCommonSharesNotIncludedInComputationOfDilutedEarningsPerShareDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/EarningsPerShareReconciliationOfBasicEarningsPerShareToDilutedEarningsPerShareDetails" xlink:href="lh-20160930.xsd#EarningsPerShareReconciliationOfBasicEarningsPerShareToDilutedEarningsPerShareDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/EarningsPerShareTables" xlink:href="lh-20160930.xsd#EarningsPerShareTables" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/FairValueMeasurements" xlink:href="lh-20160930.xsd#FairValueMeasurements" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/FairValueMeasurementsDetails" xlink:href="lh-20160930.xsd#FairValueMeasurementsDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/FairValueMeasurementsTables" xlink:href="lh-20160930.xsd#FairValueMeasurementsTables" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/GoodwillAndIntangibleAssets" xlink:href="lh-20160930.xsd#GoodwillAndIntangibleAssets" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/GoodwillAndIntangibleAssetsChangesInCarryingAmountOfGoodwillDetails" xlink:href="lh-20160930.xsd#GoodwillAndIntangibleAssetsChangesInCarryingAmountOfGoodwillDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/GoodwillAndIntangibleAssetsComponentsOfIdentifiableIntangibleAssetsDetails" xlink:href="lh-20160930.xsd#GoodwillAndIntangibleAssetsComponentsOfIdentifiableIntangibleAssetsDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/GoodwillAndIntangibleAssetsDetails" xlink:href="lh-20160930.xsd#GoodwillAndIntangibleAssetsDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/GoodwillAndIntangibleAssetsTables" xlink:href="lh-20160930.xsd#GoodwillAndIntangibleAssetsTables" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/IncomeTaxes" xlink:href="lh-20160930.xsd#IncomeTaxes" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/IncomeTaxesDetails" xlink:href="lh-20160930.xsd#IncomeTaxesDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/NoncontrollingInterestPuts" xlink:href="lh-20160930.xsd#NoncontrollingInterestPuts" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/NoncontrollingInterestPutsDetails" xlink:href="lh-20160930.xsd#NoncontrollingInterestPutsDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/PensionAndPostretirementPlans" xlink:href="lh-20160930.xsd#PensionAndPostretirementPlans" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/PensionAndPostretirementPlansDefinedContributionPlansDetails" xlink:href="lh-20160930.xsd#PensionAndPostretirementPlansDefinedContributionPlansDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/PensionAndPostretirementPlansDetails" xlink:href="lh-20160930.xsd#PensionAndPostretirementPlansDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/PensionAndPostretirementPlansTables" xlink:href="lh-20160930.xsd#PensionAndPostretirementPlansTables" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/PreferredStockAndCommonShareholdersEquity" xlink:href="lh-20160930.xsd#PreferredStockAndCommonShareholdersEquity" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/PreferredStockAndCommonShareholdersEquityDetails" xlink:href="lh-20160930.xsd#PreferredStockAndCommonShareholdersEquityDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/PreferredStockAndCommonShareholdersEquityPreferredStockAndCommonShareholdersEquityAccumulatedOtherComprehensiveEarningsDetails" xlink:href="lh-20160930.xsd#PreferredStockAndCommonShareholdersEquityPreferredStockAndCommonShareholdersEquityAccumulatedOtherComprehensiveEarningsDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/PreferredStockAndCommonShareholdersEquityTables" xlink:href="lh-20160930.xsd#PreferredStockAndCommonShareholdersEquityTables" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/RestructuringAndOtherSpecialCharges" xlink:href="lh-20160930.xsd#RestructuringAndOtherSpecialCharges" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/RestructuringAndOtherSpecialChargesDetails" xlink:href="lh-20160930.xsd#RestructuringAndOtherSpecialChargesDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/RestructuringAndOtherSpecialChargesRestructuringAndOtherSpecialChargesDetailTables" xlink:href="lh-20160930.xsd#RestructuringAndOtherSpecialChargesRestructuringAndOtherSpecialChargesDetailTables" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/RestructuringReserves" xlink:href="lh-20160930.xsd#RestructuringReserves" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/RestructuringReservesDetails" xlink:href="lh-20160930.xsd#RestructuringReservesDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/SupplementalCashFlowInformation" xlink:href="lh-20160930.xsd#SupplementalCashFlowInformation" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/SupplementalCashFlowInformationDetails" xlink:href="lh-20160930.xsd#SupplementalCashFlowInformationDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.labcorp.com/role/SupplementalCashFlowInformationTables" xlink:href="lh-20160930.xsd#SupplementalCashFlowInformationTables" xlink:type="simple" />
  <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" xlink:type="simple" />
  <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" xlink:type="simple" />
  <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" xlink:type="simple" />
  <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" xlink:type="simple" />
  <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" xlink:type="simple" />
  <definitionLink xlink:role="http://www.labcorp.com/role/BasisOfFinancialStatementPresentation" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/BasisOfFinancialStatementPresentationDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_AccountsNotesAndLoansReceivableLineItems" xlink:label="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_25BB08B77B563E6E563FE7992A5B5622" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable" xlink:label="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_404FEB473F36D400E247E7992A5B4D45" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_25BB08B77B563E6E563FE7992A5B5622" xlink:to="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_404FEB473F36D400E247E7992A5B4D45" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShortTermDebtTypeAxis" xlink:label="loc_us-gaap_ShortTermDebtTypeAxis_A44AB225D21B3DEC86D3E7992A5B3EC8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_404FEB473F36D400E247E7992A5B4D45" xlink:to="loc_us-gaap_ShortTermDebtTypeAxis_A44AB225D21B3DEC86D3E7992A5B3EC8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShortTermDebtTypeDomain" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_860F40221BEDEAF569A8E7992A5BF731_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_A44AB225D21B3DEC86D3E7992A5B3EC8" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_860F40221BEDEAF569A8E7992A5BF731_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShortTermDebtTypeDomain" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_860F40221BEDEAF569A8E7992A5BF731" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_A44AB225D21B3DEC86D3E7992A5B3EC8" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_860F40221BEDEAF569A8E7992A5BF731" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Seniornotesdue2018Member" xlink:label="loc_lh_Seniornotesdue2018Member_71A2ACA350FB83FD2A43E7992A5BA169" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_860F40221BEDEAF569A8E7992A5BF731" xlink:to="loc_lh_Seniornotesdue2018Member_71A2ACA350FB83FD2A43E7992A5BA169" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_3AA10372A18D51A76D04E7992A5B065E" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_404FEB473F36D400E247E7992A5B4D45" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_3AA10372A18D51A76D04E7992A5B065E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_8F70D5BF3C2CD284796CE7992A5B11C4_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_3AA10372A18D51A76D04E7992A5B065E" xlink:to="loc_us-gaap_SegmentDomain_8F70D5BF3C2CD284796CE7992A5B11C4_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_8F70D5BF3C2CD284796CE7992A5B11C4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_3AA10372A18D51A76D04E7992A5B065E" xlink:to="loc_us-gaap_SegmentDomain_8F70D5BF3C2CD284796CE7992A5B11C4" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_LabCorpDiagnosticsMember" xlink:label="loc_lh_LabCorpDiagnosticsMember_A7CBF7B504C44722F307E7992A5B37F6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_8F70D5BF3C2CD284796CE7992A5B11C4" xlink:to="loc_lh_LabCorpDiagnosticsMember_A7CBF7B504C44722F307E7992A5B37F6" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CovanceDrugDevelopmentMember" xlink:label="loc_lh_CovanceDrugDevelopmentMember_9DEBE9FB745090081C3DE7992A5B1B94" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_8F70D5BF3C2CD284796CE7992A5B11C4" xlink:to="loc_lh_CovanceDrugDevelopmentMember_9DEBE9FB745090081C3DE7992A5B1B94" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongTermDebtNoncurrent" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_485269F1E87E4F1038ACE7992A5BBC72" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_25BB08B77B563E6E563FE7992A5B5622" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_485269F1E87E4F1038ACE7992A5BBC72" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PriorPeriodReclassificationAdjustment" xlink:label="loc_us-gaap_PriorPeriodReclassificationAdjustment_9E66F3C923EED8048162E7992A5BF96B" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_25BB08B77B563E6E563FE7992A5B5622" xlink:to="loc_us-gaap_PriorPeriodReclassificationAdjustment_9E66F3C923EED8048162E7992A5BF96B" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_PercentofRevenueContributed" xlink:label="loc_lh_PercentofRevenueContributed_36BB5FD4B5DE4ECC43D1E7992A5B7E2E" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_25BB08B77B563E6E563FE7992A5B5622" xlink:to="loc_lh_PercentofRevenueContributed_36BB5FD4B5DE4ECC43D1E7992A5B7E2E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_24AAB80638CE91C9A8CFE7992A5BA3FC" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_25BB08B77B563E6E563FE7992A5B5622" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_24AAB80638CE91C9A8CFE7992A5BA3FC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ProceedsAndExcessTaxBenefitFromSharebasedCompensation" xlink:label="loc_us-gaap_ProceedsAndExcessTaxBenefitFromSharebasedCompensation_1C4877F90AFA797EA5D9E799E4AB2341" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_25BB08B77B563E6E563FE7992A5B5622" xlink:to="loc_us-gaap_ProceedsAndExcessTaxBenefitFromSharebasedCompensation_1C4877F90AFA797EA5D9E799E4AB2341" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOtherIncreasesDecreasesInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOtherIncreasesDecreasesInPeriod_45991F16AEB01B4F6CB4E79B9EC4C555" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_25BB08B77B563E6E563FE7992A5B5622" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOtherIncreasesDecreasesInPeriod_45991F16AEB01B4F6CB4E79B9EC4C555" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/BusinessAcquisitions" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/BusinessAcquisitionsBusinessAcquisitionsInAggregateDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_6AE2ABC47C34594AB0BB80B511CA60F8" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_106119B9D4C8526D9B8081F4B43435D6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_6AE2ABC47C34594AB0BB80B511CA60F8" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_106119B9D4C8526D9B8081F4B43435D6" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="loc_us-gaap_StatementScenarioAxis_E2275F37DB6E5F68A52DD7EC7EBE5B92" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_106119B9D4C8526D9B8081F4B43435D6" xlink:to="loc_us-gaap_StatementScenarioAxis_E2275F37DB6E5F68A52DD7EC7EBE5B92" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_9E05605B9F9B9E1DA3F3E26760FC7803_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementScenarioAxis_E2275F37DB6E5F68A52DD7EC7EBE5B92" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_9E05605B9F9B9E1DA3F3E26760FC7803_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_2C3230CCF53E5CA58861D3B8FA996257" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementScenarioAxis_E2275F37DB6E5F68A52DD7EC7EBE5B92" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_2C3230CCF53E5CA58861D3B8FA996257" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_A1397D9A4F9454E19BF079DF5CC1967D" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_106119B9D4C8526D9B8081F4B43435D6" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_A1397D9A4F9454E19BF079DF5CC1967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_A5366E13BCA999520FACE26760FC8524_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_A1397D9A4F9454E19BF079DF5CC1967D" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_A5366E13BCA999520FACE26760FC8524_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_DBFD2BB7341C5A43A6B091CFFB5533AF" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_A1397D9A4F9454E19BF079DF5CC1967D" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_DBFD2BB7341C5A43A6B091CFFB5533AF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CustomerRelationshipsMember" xlink:label="loc_us-gaap_CustomerRelationshipsMember_95C80DB214EF5BBD8A375FDB8C0410C6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_DBFD2BB7341C5A43A6B091CFFB5533AF" xlink:to="loc_us-gaap_CustomerRelationshipsMember_95C80DB214EF5BBD8A375FDB8C0410C6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_TradeNamesMember" xlink:label="loc_us-gaap_TradeNamesMember_CAD43FDAECD95C829C4098141CE3D9A6" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_DBFD2BB7341C5A43A6B091CFFB5533AF" xlink:to="loc_us-gaap_TradeNamesMember_CAD43FDAECD95C829C4098141CE3D9A6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_UseRightsMember" xlink:label="loc_us-gaap_UseRightsMember_848401AF0B68515181B59E4FCEA405C1" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_DBFD2BB7341C5A43A6B091CFFB5533AF" xlink:to="loc_us-gaap_UseRightsMember_848401AF0B68515181B59E4FCEA405C1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_4E788385C8055BE482F940C9A9CA97B5" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_106119B9D4C8526D9B8081F4B43435D6" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_4E788385C8055BE482F940C9A9CA97B5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_8C599EDFC3A1FEB213B6E26760FF08A2_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_4E788385C8055BE482F940C9A9CA97B5" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_8C599EDFC3A1FEB213B6E26760FF08A2_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2E16C601958C5320A47A9040BA5202FC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_4E788385C8055BE482F940C9A9CA97B5" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2E16C601958C5320A47A9040BA5202FC" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CovanceMember" xlink:label="loc_lh_CovanceMember_C15575081FCA5DBBAC623B4CA871681F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2E16C601958C5320A47A9040BA5202FC" xlink:to="loc_lh_CovanceMember_C15575081FCA5DBBAC623B4CA871681F" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_ExcludingCovanceMember" xlink:label="loc_lh_ExcludingCovanceMember_3FE1AE1045EA5F5BAE04D0BB5D519FBE" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2E16C601958C5320A47A9040BA5202FC" xlink:to="loc_lh_ExcludingCovanceMember_3FE1AE1045EA5F5BAE04D0BB5D519FBE" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Makewholepayment" xlink:label="loc_lh_Makewholepayment_2873D2196C795B41B5B8FE81F4D88E84" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_6AE2ABC47C34594AB0BB80B511CA60F8" xlink:to="loc_lh_Makewholepayment_2873D2196C795B41B5B8FE81F4D88E84" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Totalacquisitionconsiderationcashandnoncash" xlink:label="loc_lh_Totalacquisitionconsiderationcashandnoncash_BAA99C1B28BC582A99DDD11393CC5A34" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_6AE2ABC47C34594AB0BB80B511CA60F8" xlink:to="loc_lh_Totalacquisitionconsiderationcashandnoncash_BAA99C1B28BC582A99DDD11393CC5A34" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesLongTermDebt" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesLongTermDebt_429118067307593DADED4766A0D4B6E0" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_6AE2ABC47C34594AB0BB80B511CA60F8" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesLongTermDebt_429118067307593DADED4766A0D4B6E0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_OtherLongTermDebt" xlink:label="loc_us-gaap_OtherLongTermDebt_12803BA4645E5793B9AFFC7C8AF5B9CF" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_6AE2ABC47C34594AB0BB80B511CA60F8" xlink:to="loc_us-gaap_OtherLongTermDebt_12803BA4645E5793B9AFFC7C8AF5B9CF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_Revenues" xlink:label="loc_us-gaap_Revenues_CDD08210F31F5E858E9C3216FD846736" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_6AE2ABC47C34594AB0BB80B511CA60F8" xlink:to="loc_us-gaap_Revenues_CDD08210F31F5E858E9C3216FD846736" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_09BE501677AE5019964C7F926CF70076" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_6AE2ABC47C34594AB0BB80B511CA60F8" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_09BE501677AE5019964C7F926CF70076" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FinitelivedIntangibleAssetsAcquired1" xlink:label="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_008579414D6752AABDC560647C8EBD6D" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_6AE2ABC47C34594AB0BB80B511CA60F8" xlink:to="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_008579414D6752AABDC560647C8EBD6D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_GoodwillAcquiredDuringPeriod" xlink:label="loc_us-gaap_GoodwillAcquiredDuringPeriod_1676FC4CAA4D53F989F5DCB85D41DB96" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_6AE2ABC47C34594AB0BB80B511CA60F8" xlink:to="loc_us-gaap_GoodwillAcquiredDuringPeriod_1676FC4CAA4D53F989F5DCB85D41DB96" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_Goodwill" xlink:label="loc_us-gaap_Goodwill_793D66369DF45EDFA0A2C343240D5769" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_6AE2ABC47C34594AB0BB80B511CA60F8" xlink:to="loc_us-gaap_Goodwill_793D66369DF45EDFA0A2C343240D5769" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/BusinessAcquisitionsBusinessAcquisitionsTablesTables" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_A253AAEF00225971B207331FF63D6CC3" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_2D1ABC87A9695DE79B81251834F3D64B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_A253AAEF00225971B207331FF63D6CC3" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_2D1ABC87A9695DE79B81251834F3D64B" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_87FC1F10415B5CEDB445D9D4DF31625D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_2D1ABC87A9695DE79B81251834F3D64B" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_87FC1F10415B5CEDB445D9D4DF31625D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_008A96F969E64F302A733205F9ED079D_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_87FC1F10415B5CEDB445D9D4DF31625D" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_008A96F969E64F302A733205F9ED079D_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5475A4AC64FF5BC694C207163F0CA871" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_87FC1F10415B5CEDB445D9D4DF31625D" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5475A4AC64FF5BC694C207163F0CA871" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_09A3A4E35A4351588AD3EE3837EE121B" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_2D1ABC87A9695DE79B81251834F3D64B" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_09A3A4E35A4351588AD3EE3837EE121B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_AE39C6136BFB6215EB563205F9EDB310_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_09A3A4E35A4351588AD3EE3837EE121B" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_AE39C6136BFB6215EB563205F9EDB310_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_9209675D57EE5DE194410186B9822067" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_09A3A4E35A4351588AD3EE3837EE121B" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_9209675D57EE5DE194410186B9822067" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_37837B103CDB5A0CAA74F4B997895DC7" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_2D1ABC87A9695DE79B81251834F3D64B" xlink:to="loc_us-gaap_StatementClassOfStockAxis_37837B103CDB5A0CAA74F4B997895DC7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_F6BF96460F8086E7E14F3205F9EDBB69_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_37837B103CDB5A0CAA74F4B997895DC7" xlink:to="loc_us-gaap_ClassOfStockDomain_F6BF96460F8086E7E14F3205F9EDBB69_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_73FDC4151BE559E396349C71A9178DE1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_37837B103CDB5A0CAA74F4B997895DC7" xlink:to="loc_us-gaap_ClassOfStockDomain_73FDC4151BE559E396349C71A9178DE1" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/BusinessAcquisitionsDetails" xlink:type="extended">
    <loc xlink:href="lh-20160930.xsd#lh_BusinessCombinationPurchasePriceAllocationLineItems" xlink:label="loc_lh_BusinessCombinationPurchasePriceAllocationLineItems_4F52542B1DF2D0469885FD622437DBEE" xlink:type="locator" />
    <loc xlink:href="lh-20160930.xsd#lh_BusinessCombinationPurchasePriceAllocationTable" xlink:label="loc_lh_BusinessCombinationPurchasePriceAllocationTable_0A7EDD4B91D033AB93F3FD62243703D7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationLineItems_4F52542B1DF2D0469885FD622437DBEE" xlink:to="loc_lh_BusinessCombinationPurchasePriceAllocationTable_0A7EDD4B91D033AB93F3FD62243703D7" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_16D6CFF6AB84614FB7AEFD6224377255" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationTable_0A7EDD4B91D033AB93F3FD62243703D7" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_16D6CFF6AB84614FB7AEFD6224377255" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_8234A34F90365EBAB2A8FD62243706C3_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_16D6CFF6AB84614FB7AEFD6224377255" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_8234A34F90365EBAB2A8FD62243706C3_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_8234A34F90365EBAB2A8FD62243706C3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_16D6CFF6AB84614FB7AEFD6224377255" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_8234A34F90365EBAB2A8FD62243706C3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_UseRightsMember" xlink:label="loc_us-gaap_UseRightsMember_B8F11BBA17836F04ABC0FD622437BC3C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_8234A34F90365EBAB2A8FD62243706C3" xlink:to="loc_us-gaap_UseRightsMember_B8F11BBA17836F04ABC0FD622437BC3C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CustomerRelationshipsMember" xlink:label="loc_us-gaap_CustomerRelationshipsMember_5CB826F6F1029DC30283FD622437B31F" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_8234A34F90365EBAB2A8FD62243706C3" xlink:to="loc_us-gaap_CustomerRelationshipsMember_5CB826F6F1029DC30283FD622437B31F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="loc_us-gaap_StatementScenarioAxis_BF80794F3F0C28B5CCA2FD622437D37D" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationTable_0A7EDD4B91D033AB93F3FD62243703D7" xlink:to="loc_us-gaap_StatementScenarioAxis_BF80794F3F0C28B5CCA2FD622437D37D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_1D77C20D764DBF7AC20BFD622437709C_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementScenarioAxis_BF80794F3F0C28B5CCA2FD622437D37D" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_1D77C20D764DBF7AC20BFD622437709C_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_1D77C20D764DBF7AC20BFD622437709C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementScenarioAxis_BF80794F3F0C28B5CCA2FD622437D37D" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_1D77C20D764DBF7AC20BFD622437709C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_2482A15C0CB92DB1605BFD62243797F2" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationTable_0A7EDD4B91D033AB93F3FD62243703D7" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_2482A15C0CB92DB1605BFD62243797F2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_16E3AEF458C81406980EFD622437E9EC_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_2482A15C0CB92DB1605BFD62243797F2" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_16E3AEF458C81406980EFD622437E9EC_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_16E3AEF458C81406980EFD622437E9EC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_2482A15C0CB92DB1605BFD62243797F2" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_16E3AEF458C81406980EFD622437E9EC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SubsequentEventMember" xlink:label="loc_us-gaap_SubsequentEventMember_E926096BCA1B4F52452CFD6224376821" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_16E3AEF458C81406980EFD622437E9EC" xlink:to="loc_us-gaap_SubsequentEventMember_E926096BCA1B4F52452CFD6224376821" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_6A4EB0875BF04C208325FD6224372328" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationTable_0A7EDD4B91D033AB93F3FD62243703D7" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_6A4EB0875BF04C208325FD6224372328" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_D8042B4F50FC1A5A1C4AFD6224376202_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_6A4EB0875BF04C208325FD6224372328" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_D8042B4F50FC1A5A1C4AFD6224376202_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_D8042B4F50FC1A5A1C4AFD6224376202" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_6A4EB0875BF04C208325FD6224372328" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_D8042B4F50FC1A5A1C4AFD6224376202" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CovanceMember" xlink:label="loc_lh_CovanceMember_E9020E5F267192989173FD6224372277" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_D8042B4F50FC1A5A1C4AFD6224376202" xlink:to="loc_lh_CovanceMember_E9020E5F267192989173FD6224372277" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_DB8051A5D58E58E44628FD62243750AD" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationTable_0A7EDD4B91D033AB93F3FD62243703D7" xlink:to="loc_us-gaap_StatementClassOfStockAxis_DB8051A5D58E58E44628FD62243750AD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_DF48744DB3EBDEAD93C9FD6224376E2A_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_DB8051A5D58E58E44628FD62243750AD" xlink:to="loc_us-gaap_ClassOfStockDomain_DF48744DB3EBDEAD93C9FD6224376E2A_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_DF48744DB3EBDEAD93C9FD6224376E2A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_DB8051A5D58E58E44628FD62243750AD" xlink:to="loc_us-gaap_ClassOfStockDomain_DF48744DB3EBDEAD93C9FD6224376E2A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_ACB663E874CB04841212FD622437997E" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationTable_0A7EDD4B91D033AB93F3FD62243703D7" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_ACB663E874CB04841212FD622437997E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_0243EAF410D8DBFD46D1FD6224373276_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_ACB663E874CB04841212FD622437997E" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_0243EAF410D8DBFD46D1FD6224373276_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_0243EAF410D8DBFD46D1FD6224373276" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_ACB663E874CB04841212FD622437997E" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_0243EAF410D8DBFD46D1FD6224373276" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_InterestExpenseMember" xlink:label="loc_us-gaap_InterestExpenseMember_5EFE482A246B05F72071FD6224370F1C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_0243EAF410D8DBFD46D1FD6224373276" xlink:to="loc_us-gaap_InterestExpenseMember_5EFE482A246B05F72071FD6224370F1C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_Revenues" xlink:label="loc_us-gaap_Revenues_1087446F608D58A89239FD622437AC5C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationLineItems_4F52542B1DF2D0469885FD622437DBEE" xlink:to="loc_us-gaap_Revenues_1087446F608D58A89239FD622437AC5C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesLongTermDebt" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesLongTermDebt_1F9661703AC4D659092FFD622437DE72" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationLineItems_4F52542B1DF2D0469885FD622437DBEE" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesLongTermDebt_1F9661703AC4D659092FFD622437DE72" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_OperatingIncomeLoss" xlink:label="loc_us-gaap_OperatingIncomeLoss_5C88017EBA9B3FB9633BFD6224377956" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationLineItems_4F52542B1DF2D0469885FD622437DBEE" xlink:to="loc_us-gaap_OperatingIncomeLoss_5C88017EBA9B3FB9633BFD6224377956" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_OtherLongTermDebt" xlink:label="loc_us-gaap_OtherLongTermDebt_922002447A15689AB7D3FD62243738BF" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationLineItems_4F52542B1DF2D0469885FD622437DBEE" xlink:to="loc_us-gaap_OtherLongTermDebt_922002447A15689AB7D3FD62243738BF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_OtherSignificantNoncashTransactionValueOfConsiderationGiven1" xlink:label="loc_us-gaap_OtherSignificantNoncashTransactionValueOfConsiderationGiven1_9927DAF3AE710DBA62D1FD622437DF86" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationLineItems_4F52542B1DF2D0469885FD622437DBEE" xlink:to="loc_us-gaap_OtherSignificantNoncashTransactionValueOfConsiderationGiven1_9927DAF3AE710DBA62D1FD622437DF86" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_E9A46B4B8B4E264C4936FD6224374BA3" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationLineItems_4F52542B1DF2D0469885FD622437DBEE" xlink:to="loc_us-gaap_NetIncomeLoss_E9A46B4B8B4E264C4936FD6224374BA3" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Totalacquisitionconsiderationcashandnoncash" xlink:label="loc_lh_Totalacquisitionconsiderationcashandnoncash_94538EFA7802EE637D32FD622437CA2E" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationLineItems_4F52542B1DF2D0469885FD622437DBEE" xlink:to="loc_lh_Totalacquisitionconsiderationcashandnoncash_94538EFA7802EE637D32FD622437CA2E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_GoodwillAcquiredDuringPeriod" xlink:label="loc_us-gaap_GoodwillAcquiredDuringPeriod_00AE7C650A064D10383FFD622437871A" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationLineItems_4F52542B1DF2D0469885FD622437DBEE" xlink:to="loc_us-gaap_GoodwillAcquiredDuringPeriod_00AE7C650A064D10383FFD622437871A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SeniorLongTermNotes" xlink:label="loc_us-gaap_SeniorLongTermNotes_C65F4A359E60D70CFE17FD6224372DE1" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationLineItems_4F52542B1DF2D0469885FD622437DBEE" xlink:to="loc_us-gaap_SeniorLongTermNotes_C65F4A359E60D70CFE17FD6224372DE1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_Goodwill" xlink:label="loc_us-gaap_Goodwill_3A50BFFAFC6A3B80FBBDFD6224376D8E" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationLineItems_4F52542B1DF2D0469885FD622437DBEE" xlink:to="loc_us-gaap_Goodwill_3A50BFFAFC6A3B80FBBDFD6224376D8E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_558D5CD6C16E546B360AFD622437892C" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationLineItems_4F52542B1DF2D0469885FD622437DBEE" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_558D5CD6C16E546B360AFD622437892C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_310BA5FB80CC0A8C3368FD6224374923" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationLineItems_4F52542B1DF2D0469885FD622437DBEE" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_310BA5FB80CC0A8C3368FD6224374923" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_3505CE15FE8C2BE3D8ECFD622437B174" xlink:type="locator" />
    <definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationLineItems_4F52542B1DF2D0469885FD622437DBEE" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_3505CE15FE8C2BE3D8ECFD622437B174" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FinitelivedIntangibleAssetsAcquired1" xlink:label="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_C1B4D33C8C2BA485AE7EFD622437FFD8" xlink:type="locator" />
    <definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationLineItems_4F52542B1DF2D0469885FD622437DBEE" xlink:to="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_C1B4D33C8C2BA485AE7EFD622437FFD8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionSharePrice" xlink:label="loc_us-gaap_BusinessAcquisitionSharePrice_206F2DF647E7BA50D156FD622437EBD4" xlink:type="locator" />
    <definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_BusinessCombinationPurchasePriceAllocationLineItems_4F52542B1DF2D0469885FD622437DBEE" xlink:to="loc_us-gaap_BusinessAcquisitionSharePrice_206F2DF647E7BA50D156FD622437EBD4" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/BusinessSegmentInformationBusinessSegmentInformationDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_98A92D3A7E5B5336AAB354BF7456DC90" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_ACB6818C606B54D19463C9B12499F69C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_98A92D3A7E5B5336AAB354BF7456DC90" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_ACB6818C606B54D19463C9B12499F69C" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_B6583CA75CF65008A7CB17322CAAB94C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_ACB6818C606B54D19463C9B12499F69C" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_B6583CA75CF65008A7CB17322CAAB94C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_2F44F8081FADA4F1832032D9D4ED314E_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_B6583CA75CF65008A7CB17322CAAB94C" xlink:to="loc_us-gaap_SegmentDomain_2F44F8081FADA4F1832032D9D4ED314E_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_3D7A06704ED8503A87A4A95BE84FBD4A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_B6583CA75CF65008A7CB17322CAAB94C" xlink:to="loc_us-gaap_SegmentDomain_3D7A06704ED8503A87A4A95BE84FBD4A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CorporateMember" xlink:label="loc_us-gaap_CorporateMember_0F05353630BD50AFBF7611B52AC943BB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_3D7A06704ED8503A87A4A95BE84FBD4A" xlink:to="loc_us-gaap_CorporateMember_0F05353630BD50AFBF7611B52AC943BB" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_LabCorpDiagnosticsMember" xlink:label="loc_lh_LabCorpDiagnosticsMember_AFE4470204265966845D61EEB4B2721D" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_3D7A06704ED8503A87A4A95BE84FBD4A" xlink:to="loc_lh_LabCorpDiagnosticsMember_AFE4470204265966845D61EEB4B2721D" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CovanceDrugDevelopmentMember" xlink:label="loc_lh_CovanceDrugDevelopmentMember_D24753874694563E901613A73F666D40" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_3D7A06704ED8503A87A4A95BE84FBD4A" xlink:to="loc_lh_CovanceDrugDevelopmentMember_D24753874694563E901613A73F666D40" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_PercentofRevenueContributed" xlink:label="loc_lh_PercentofRevenueContributed_5A32C767251555CC8D01CD7993B2631A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_98A92D3A7E5B5336AAB354BF7456DC90" xlink:to="loc_lh_PercentofRevenueContributed_5A32C767251555CC8D01CD7993B2631A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SalesRevenueNet" xlink:label="loc_us-gaap_SalesRevenueNet_D13619A3BC04591B961718C98F9B704E" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_98A92D3A7E5B5336AAB354BF7456DC90" xlink:to="loc_us-gaap_SalesRevenueNet_D13619A3BC04591B961718C98F9B704E" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Intercompanyrevenueelimination" xlink:label="loc_lh_Intercompanyrevenueelimination_AE78FE3B69855E088D9DF5E6E073634F" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_98A92D3A7E5B5336AAB354BF7456DC90" xlink:to="loc_lh_Intercompanyrevenueelimination_AE78FE3B69855E088D9DF5E6E073634F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ReimbursementRevenue" xlink:label="loc_us-gaap_ReimbursementRevenue_D00C59BA5225558EA70B0127269B95DC" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_98A92D3A7E5B5336AAB354BF7456DC90" xlink:to="loc_us-gaap_ReimbursementRevenue_D00C59BA5225558EA70B0127269B95DC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_Revenues" xlink:label="loc_us-gaap_Revenues_D724800FA4E050D28FFACB1EFB0F5F1E" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_98A92D3A7E5B5336AAB354BF7456DC90" xlink:to="loc_us-gaap_Revenues_D724800FA4E050D28FFACB1EFB0F5F1E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_OperatingIncomeLoss" xlink:label="loc_us-gaap_OperatingIncomeLoss_7D5B133C3D7651CAA1DF1FBA7568BEB5" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_98A92D3A7E5B5336AAB354BF7456DC90" xlink:to="loc_us-gaap_OperatingIncomeLoss_7D5B133C3D7651CAA1DF1FBA7568BEB5" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_IncomeLossfromContinuingOperationsBeforeIncomeTaxesandNoncontrollingInterest" xlink:label="loc_lh_IncomeLossfromContinuingOperationsBeforeIncomeTaxesandNoncontrollingInterest_642E226FDA945296A7CD7D2378EB6971" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_98A92D3A7E5B5336AAB354BF7456DC90" xlink:to="loc_lh_IncomeLossfromContinuingOperationsBeforeIncomeTaxesandNoncontrollingInterest_642E226FDA945296A7CD7D2378EB6971" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_IncomeLossFromContinuingOperationsBeforeIncomeTaxesAndMinorityInterest" xlink:label="loc_lh_IncomeLossFromContinuingOperationsBeforeIncomeTaxesAndMinorityInterest_901252ED25025A9F8D2B32F8FE416A82" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_98A92D3A7E5B5336AAB354BF7456DC90" xlink:to="loc_lh_IncomeLossFromContinuingOperationsBeforeIncomeTaxesAndMinorityInterest_901252ED25025A9F8D2B32F8FE416A82" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_F1E3C1285474518F9A459D13D30AB8F3" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_98A92D3A7E5B5336AAB354BF7456DC90" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_F1E3C1285474518F9A459D13D30AB8F3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ProfitLoss" xlink:label="loc_us-gaap_ProfitLoss_64C7272491A3594BB73EC3F36DB7594F" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_98A92D3A7E5B5336AAB354BF7456DC90" xlink:to="loc_us-gaap_ProfitLoss_64C7272491A3594BB73EC3F36DB7594F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_NetIncomeLossAttributableToNoncontrollingInterest" xlink:label="loc_us-gaap_NetIncomeLossAttributableToNoncontrollingInterest_CB313C7CBDBD5C329DF34D77F31F1AA3" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_98A92D3A7E5B5336AAB354BF7456DC90" xlink:to="loc_us-gaap_NetIncomeLossAttributableToNoncontrollingInterest_CB313C7CBDBD5C329DF34D77F31F1AA3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_NonoperatingIncomeExpense" xlink:label="loc_us-gaap_NonoperatingIncomeExpense_459FD6DC3D605A668E1294AE2692727B" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_98A92D3A7E5B5336AAB354BF7456DC90" xlink:to="loc_us-gaap_NonoperatingIncomeExpense_459FD6DC3D605A668E1294AE2692727B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_F94451D44DA850299F02A36B6E17A44F" xlink:type="locator" />
    <definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_98A92D3A7E5B5336AAB354BF7456DC90" xlink:to="loc_us-gaap_NetIncomeLoss_F94451D44DA850299F02A36B6E17A44F" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/BusinessSegmentInformationBusinessSegmentInformationNotes" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_71121BBDFC2A5EDDAAFB7158532669AD" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_4E8965DCDD2D547E93D6A0533D101A32" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_71121BBDFC2A5EDDAAFB7158532669AD" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_4E8965DCDD2D547E93D6A0533D101A32" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_AB7081576B7A5842981C9218F2B8FB98" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_4E8965DCDD2D547E93D6A0533D101A32" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_AB7081576B7A5842981C9218F2B8FB98" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_A71126051FE1F4FC716832D9D78BB131_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_AB7081576B7A5842981C9218F2B8FB98" xlink:to="loc_us-gaap_SegmentDomain_A71126051FE1F4FC716832D9D78BB131_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_695B6794195C55F8B6E9E69A42411DA8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_AB7081576B7A5842981C9218F2B8FB98" xlink:to="loc_us-gaap_SegmentDomain_695B6794195C55F8B6E9E69A42411DA8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentReportingDisclosureTextBlock" xlink:label="loc_us-gaap_SegmentReportingDisclosureTextBlock_3A42C77F98E855228E4E55B1152ADD3F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_71121BBDFC2A5EDDAAFB7158532669AD" xlink:to="loc_us-gaap_SegmentReportingDisclosureTextBlock_3A42C77F98E855228E4E55B1152ADD3F" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/BusinessSegmentInformationBusinessSegmentInformationSegmentReconciliationOfOperatingIncomeToConsolidatedTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/CommitmentsAndContingencies" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/CommitmentsAndContingenciesDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LossContingenciesLineItems" xlink:label="loc_us-gaap_LossContingenciesLineItems_1F53DF99DAE4FB969E8D07B387E42B21" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LossContingenciesTable" xlink:label="loc_us-gaap_LossContingenciesTable_F495D0AAC153B12DB2B407B387E34332" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_1F53DF99DAE4FB969E8D07B387E42B21" xlink:to="loc_us-gaap_LossContingenciesTable_F495D0AAC153B12DB2B407B387E34332" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_964F5AB5E752EB635A9907B387E3B11E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_F495D0AAC153B12DB2B407B387E34332" xlink:to="loc_us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_964F5AB5E752EB635A9907B387E3B11E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_EquityMethodInvesteeNameDomain" xlink:label="loc_us-gaap_EquityMethodInvesteeNameDomain_E5B6A246EE039DED142F07B387E34CB7_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_964F5AB5E752EB635A9907B387E3B11E" xlink:to="loc_us-gaap_EquityMethodInvesteeNameDomain_E5B6A246EE039DED142F07B387E34CB7_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_EquityMethodInvesteeNameDomain" xlink:label="loc_us-gaap_EquityMethodInvesteeNameDomain_E5B6A246EE039DED142F07B387E34CB7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_964F5AB5E752EB635A9907B387E3B11E" xlink:to="loc_us-gaap_EquityMethodInvesteeNameDomain_E5B6A246EE039DED142F07B387E34CB7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_C053C89C6DD4C0DCB0E507B387E34EA8" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_F495D0AAC153B12DB2B407B387E34332" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_C053C89C6DD4C0DCB0E507B387E34EA8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_DF17602D724CFAD3C10407B387E38EDF_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_C053C89C6DD4C0DCB0E507B387E34EA8" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_DF17602D724CFAD3C10407B387E38EDF_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_DF17602D724CFAD3C10407B387E38EDF" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_C053C89C6DD4C0DCB0E507B387E34EA8" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_DF17602D724CFAD3C10407B387E38EDF" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CovanceMember" xlink:label="loc_lh_CovanceMember_485C7455E500948DEF9207B387E33206" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_DF17602D724CFAD3C10407B387E38EDF" xlink:to="loc_lh_CovanceMember_485C7455E500948DEF9207B387E33206" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_A923F8C11AE2B311AC2E07B387E35978" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_F495D0AAC153B12DB2B407B387E34332" xlink:to="loc_us-gaap_RangeAxis_A923F8C11AE2B311AC2E07B387E35978" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_717124CB052C5FB787C207B387E44B77_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_A923F8C11AE2B311AC2E07B387E35978" xlink:to="loc_us-gaap_RangeMember_717124CB052C5FB787C207B387E44B77_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_717124CB052C5FB787C207B387E44B77" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_A923F8C11AE2B311AC2E07B387E35978" xlink:to="loc_us-gaap_RangeMember_717124CB052C5FB787C207B387E44B77" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_48D1767C9FA1F022295A07B387E43831" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_717124CB052C5FB787C207B387E44B77" xlink:to="loc_us-gaap_MaximumMember_48D1767C9FA1F022295A07B387E43831" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_8B7B7232BA775888377307B387E4F2B5" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_F495D0AAC153B12DB2B407B387E34332" xlink:to="loc_dei_LegalEntityAxis_8B7B7232BA775888377307B387E4F2B5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_360490B3D2809AA8582507B387E4120E_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_8B7B7232BA775888377307B387E4F2B5" xlink:to="loc_dei_EntityDomain_360490B3D2809AA8582507B387E4120E_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_360490B3D2809AA8582507B387E4120E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_8B7B7232BA775888377307B387E4F2B5" xlink:to="loc_dei_EntityDomain_360490B3D2809AA8582507B387E4120E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_GainLossRelatedToLitigationSettlement" xlink:label="loc_us-gaap_GainLossRelatedToLitigationSettlement_EFEDAD4E85D64A998ECC07B387E4DB84" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_1F53DF99DAE4FB969E8D07B387E42B21" xlink:to="loc_us-gaap_GainLossRelatedToLitigationSettlement_EFEDAD4E85D64A998ECC07B387E4DB84" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_PreviouslyRecordedLitigationReserveInConnectionWithFalseClaimsActLawsuit" xlink:label="loc_lh_PreviouslyRecordedLitigationReserveInConnectionWithFalseClaimsActLawsuit_C33B516871AA867F093307B387E4ED1C" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_1F53DF99DAE4FB969E8D07B387E42B21" xlink:to="loc_lh_PreviouslyRecordedLitigationReserveInConnectionWithFalseClaimsActLawsuit_C33B516871AA867F093307B387E4ED1C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LitigationSettlementAmount" xlink:label="loc_us-gaap_LitigationSettlementAmount_6CD338628B24707DABDF07B387E4F4DB" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_1F53DF99DAE4FB969E8D07B387E42B21" xlink:to="loc_us-gaap_LitigationSettlementAmount_6CD338628B24707DABDF07B387E4F4DB" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_PaymentreductionbyAssemblyBillNo.97" xlink:label="loc_lh_PaymentreductionbyAssemblyBillNo.97_CF720012EF99D6C2CFE707B387E44666" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_1F53DF99DAE4FB969E8D07B387E42B21" xlink:to="loc_lh_PaymentreductionbyAssemblyBillNo.97_CF720012EF99D6C2CFE707B387E44666" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Percentoflowestmaximumcaponpayments" xlink:label="loc_lh_Percentoflowestmaximumcaponpayments_E27814D450B605BAE1DB07B387E4BCB7" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_1F53DF99DAE4FB969E8D07B387E42B21" xlink:to="loc_lh_Percentoflowestmaximumcaponpayments_E27814D450B605BAE1DB07B387E4BCB7" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Totalacquisitionconsiderationcashandnoncash" xlink:label="loc_lh_Totalacquisitionconsiderationcashandnoncash_1337DE9171976735FB9B07B387E40433" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_1F53DF99DAE4FB969E8D07B387E42B21" xlink:to="loc_lh_Totalacquisitionconsiderationcashandnoncash_1337DE9171976735FB9B07B387E40433" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_NumberOfRecipients" xlink:label="loc_lh_NumberOfRecipients_A5895A9C5B9938ABEB1E07B387E46C2F" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_1F53DF99DAE4FB969E8D07B387E42B21" xlink:to="loc_lh_NumberOfRecipients_A5895A9C5B9938ABEB1E07B387E46C2F" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_ProposedDamagesPerViolation" xlink:label="loc_lh_ProposedDamagesPerViolation_04B38297FB9FEAC3165207B387E450BB" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_1F53DF99DAE4FB969E8D07B387E42B21" xlink:to="loc_lh_ProposedDamagesPerViolation_04B38297FB9FEAC3165207B387E450BB" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_AHStransfertoCanadianJV" xlink:label="loc_lh_AHStransfertoCanadianJV_E9392EF36D1560B6605507B57FEECD95" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_1F53DF99DAE4FB969E8D07B387E42B21" xlink:to="loc_lh_AHStransfertoCanadianJV_E9392EF36D1560B6605507B57FEECD95" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_AmountOutstandingLettersOfCredit" xlink:label="loc_lh_AmountOutstandingLettersOfCredit_8D2B9F8BDDE410DACA7607B387E4F9D2" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_1F53DF99DAE4FB969E8D07B387E42B21" xlink:to="loc_lh_AmountOutstandingLettersOfCredit_8D2B9F8BDDE410DACA7607B387E4F9D2" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/CondensedConsolidatedBalanceSheetsUnaudited" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/CondensedConsolidatedBalanceSheetsUnauditedParentheticals" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/CondensedConsolidatedStatementsOfCashFlowsUnaudited" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_96D5BE7D26A7A19D6874E81A4922A61A" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_D2F761A22151987426E4E81A49228FD3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_96D5BE7D26A7A19D6874E81A4922A61A" xlink:to="loc_us-gaap_StatementTable_D2F761A22151987426E4E81A49228FD3" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="loc_us-gaap_StatementScenarioAxis_3D4F652A47493BFA82E5E81A49228747" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_D2F761A22151987426E4E81A49228FD3" xlink:to="loc_us-gaap_StatementScenarioAxis_3D4F652A47493BFA82E5E81A49228747" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_EE37C0FA5DABB6768B02E81A4922947D_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementScenarioAxis_3D4F652A47493BFA82E5E81A49228747" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_EE37C0FA5DABB6768B02E81A4922947D_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_EE37C0FA5DABB6768B02E81A4922947D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementScenarioAxis_3D4F652A47493BFA82E5E81A49228747" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_EE37C0FA5DABB6768B02E81A4922947D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ProfitLoss" xlink:label="loc_us-gaap_ProfitLoss_BDA052219B4054AA3EC8E81A49227114" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_96D5BE7D26A7A19D6874E81A4922A61A" xlink:to="loc_us-gaap_ProfitLoss_BDA052219B4054AA3EC8E81A49227114" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_IncomeLossfromContinuingOperationsBeforeIncomeTaxesandNoncontrollingInterest" xlink:label="loc_lh_IncomeLossfromContinuingOperationsBeforeIncomeTaxesandNoncontrollingInterest_FA86B9425FCA6D78ED37E81A4922A84B" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_96D5BE7D26A7A19D6874E81A4922A61A" xlink:to="loc_lh_IncomeLossfromContinuingOperationsBeforeIncomeTaxesandNoncontrollingInterest_FA86B9425FCA6D78ED37E81A4922A84B" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Paymentsonbridgeloan" xlink:label="loc_lh_Paymentsonbridgeloan_72749BBBD162E21C5DCCE81A492292F6" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_96D5BE7D26A7A19D6874E81A4922A61A" xlink:to="loc_lh_Paymentsonbridgeloan_72749BBBD162E21C5DCCE81A492292F6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RepaymentsOfSeniorDebt" xlink:label="loc_us-gaap_RepaymentsOfSeniorDebt_DC40219BD5FE47A0CC43E81A4922DB7E" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_96D5BE7D26A7A19D6874E81A4922A61A" xlink:to="loc_us-gaap_RepaymentsOfSeniorDebt_DC40219BD5FE47A0CC43E81A4922DB7E" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Proceedsfrombridgeloan" xlink:label="loc_lh_Proceedsfrombridgeloan_9435CA63D37805EBB7F0E81A4922C8C8" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_96D5BE7D26A7A19D6874E81A4922A61A" xlink:to="loc_lh_Proceedsfrombridgeloan_9435CA63D37805EBB7F0E81A4922C8C8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ProceedsFromIssuanceOfOtherLongTermDebt" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfOtherLongTermDebt_C4FA0433D5F08D2D1321E81A4922C71B" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_96D5BE7D26A7A19D6874E81A4922A61A" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfOtherLongTermDebt_C4FA0433D5F08D2D1321E81A4922C71B" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_AccretedInterestOnZeroCouponSubordinatedNotes" xlink:label="loc_lh_AccretedInterestOnZeroCouponSubordinatedNotes_F291A60E74D99589617EE81A49224A20" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_96D5BE7D26A7A19D6874E81A4922A61A" xlink:to="loc_lh_AccretedInterestOnZeroCouponSubordinatedNotes_F291A60E74D99589617EE81A49224A20" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ProceedsFromSaleOfEquityMethodInvestments" xlink:label="loc_us-gaap_ProceedsFromSaleOfEquityMethodInvestments_D5FDEF6082B24182DC73E81A4922BD84" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_96D5BE7D26A7A19D6874E81A4922A61A" xlink:to="loc_us-gaap_ProceedsFromSaleOfEquityMethodInvestments_D5FDEF6082B24182DC73E81A4922BD84" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_PaymentsOnZeroCouponSubordinatedNotes" xlink:label="loc_lh_PaymentsOnZeroCouponSubordinatedNotes_2344E3579677116649E5E81ADED3D6BF" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_96D5BE7D26A7A19D6874E81A4922A61A" xlink:to="loc_lh_PaymentsOnZeroCouponSubordinatedNotes_2344E3579677116649E5E81ADED3D6BF" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/CondensedConsolidatedStatementsOfChangesInShareholdersEquityUnaudited" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_2F97CE8BE88957868891D87FB6093B6A" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_EE13FD6566995E3F831215933B142CE3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_2F97CE8BE88957868891D87FB6093B6A" xlink:to="loc_us-gaap_StatementTable_EE13FD6566995E3F831215933B142CE3" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_3A33C6F31CE853AE9EA13EA79DA6CA43" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_EE13FD6566995E3F831215933B142CE3" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_3A33C6F31CE853AE9EA13EA79DA6CA43" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_E7C6CEBF4B28EFC179AD32A6490792CE_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_3A33C6F31CE853AE9EA13EA79DA6CA43" xlink:to="loc_us-gaap_EquityComponentDomain_E7C6CEBF4B28EFC179AD32A6490792CE_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_BDD0ED1722E6555F9F28BECDBE8B75E3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_3A33C6F31CE853AE9EA13EA79DA6CA43" xlink:to="loc_us-gaap_EquityComponentDomain_BDD0ED1722E6555F9F28BECDBE8B75E3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CommonStockMember" xlink:label="loc_us-gaap_CommonStockMember_32A32009254C585293233E67ADD0415F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_BDD0ED1722E6555F9F28BECDBE8B75E3" xlink:to="loc_us-gaap_CommonStockMember_32A32009254C585293233E67ADD0415F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_2E944B70245C5339B87338200AAEF19D" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_BDD0ED1722E6555F9F28BECDBE8B75E3" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_2E944B70245C5339B87338200AAEF19D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RetainedEarningsMember" xlink:label="loc_us-gaap_RetainedEarningsMember_A41430B20DE85300869717F23ACD4EDA" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_BDD0ED1722E6555F9F28BECDBE8B75E3" xlink:to="loc_us-gaap_RetainedEarningsMember_A41430B20DE85300869717F23ACD4EDA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_TreasuryStockMember" xlink:label="loc_us-gaap_TreasuryStockMember_147BA7B349AA5AE48E94E5C41A03808F" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_BDD0ED1722E6555F9F28BECDBE8B75E3" xlink:to="loc_us-gaap_TreasuryStockMember_147BA7B349AA5AE48E94E5C41A03808F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_F22370EFB1D2538C87DC5A6B7074C4BE" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_BDD0ED1722E6555F9F28BECDBE8B75E3" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_F22370EFB1D2538C87DC5A6B7074C4BE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StockholdersEquity" xlink:label="loc_us-gaap_StockholdersEquity_4E611A71C008509A9FCF91DF832097A2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_2F97CE8BE88957868891D87FB6093B6A" xlink:to="loc_us-gaap_StockholdersEquity_4E611A71C008509A9FCF91DF832097A2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_4E80F264E4B75AA4ACF04F2C6F4DC7E4" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_2F97CE8BE88957868891D87FB6093B6A" xlink:to="loc_us-gaap_NetIncomeLoss_4E80F264E4B75AA4ACF04F2C6F4DC7E4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_2E08C49170565260A93C56065C1EC8E8" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_2F97CE8BE88957868891D87FB6093B6A" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_2E08C49170565260A93C56065C1EC8E8" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Issuanceofcommonstockforacquisitionconsideration" xlink:label="loc_lh_Issuanceofcommonstockforacquisitionconsideration_48EB9C87B831572F9F28802F7AEB051C" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_2F97CE8BE88957868891D87FB6093B6A" xlink:to="loc_lh_Issuanceofcommonstockforacquisitionconsideration_48EB9C87B831572F9F28802F7AEB051C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_OtherSignificantNoncashTransactionValueOfConsiderationGiven1" xlink:label="loc_us-gaap_OtherSignificantNoncashTransactionValueOfConsiderationGiven1_6A9A130876B65021A0A6C13B4355F49E" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_2F97CE8BE88957868891D87FB6093B6A" xlink:to="loc_us-gaap_OtherSignificantNoncashTransactionValueOfConsiderationGiven1_6A9A130876B65021A0A6C13B4355F49E" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Issuanceofcommonstockunderemployeeplans" xlink:label="loc_lh_Issuanceofcommonstockunderemployeeplans_B7ADF9D70EE45C24BD6D75849A65CA78" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_2F97CE8BE88957868891D87FB6093B6A" xlink:to="loc_lh_Issuanceofcommonstockunderemployeeplans_B7ADF9D70EE45C24BD6D75849A65CA78" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures_82020FB79D945F888B40BB0456896212" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_2F97CE8BE88957868891D87FB6093B6A" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures_82020FB79D945F888B40BB0456896212" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Conversionofconvertibledebt" xlink:label="loc_lh_Conversionofconvertibledebt_A359497CE4255CA8A28B524DC488AF65" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_2F97CE8BE88957868891D87FB6093B6A" xlink:to="loc_lh_Conversionofconvertibledebt_A359497CE4255CA8A28B524DC488AF65" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_3A93A3D3AFEF54278B63E622C4273BFE" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_2F97CE8BE88957868891D87FB6093B6A" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_3A93A3D3AFEF54278B63E622C4273BFE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation_DA3CFE685D0A5AE49F4168FE338EC34B" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_2F97CE8BE88957868891D87FB6093B6A" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation_DA3CFE685D0A5AE49F4168FE338EC34B" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/CondensedConsolidatedStatementsOfComprehensiveEarningsUnaudited" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_8B8AB6D0D397C480F0AEE764214CA433" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_B2A629EFDCBD6FFAA5F6E764214CE294" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_8B8AB6D0D397C480F0AEE764214CA433" xlink:to="loc_us-gaap_StatementTable_B2A629EFDCBD6FFAA5F6E764214CE294" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="loc_us-gaap_StatementScenarioAxis_BB34E1294E34028A3FACE764214C8165" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_B2A629EFDCBD6FFAA5F6E764214CE294" xlink:to="loc_us-gaap_StatementScenarioAxis_BB34E1294E34028A3FACE764214C8165" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_FB8D19F82476A2D350FCE764214C9A11_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementScenarioAxis_BB34E1294E34028A3FACE764214C8165" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_FB8D19F82476A2D350FCE764214C9A11_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_FB8D19F82476A2D350FCE764214C9A11" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementScenarioAxis_BB34E1294E34028A3FACE764214C8165" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_FB8D19F82476A2D350FCE764214C9A11" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SalesRevenueNet" xlink:label="loc_us-gaap_SalesRevenueNet_AFB27985C40A8578D765E764214C9134" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_8B8AB6D0D397C480F0AEE764214CA433" xlink:to="loc_us-gaap_SalesRevenueNet_AFB27985C40A8578D765E764214C9134" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_NetRestructuringAndOtherSpecialCharges" xlink:label="loc_lh_NetRestructuringAndOtherSpecialCharges_9C4E536AEB0F29843F66E764214C370B" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_8B8AB6D0D397C480F0AEE764214CA433" xlink:to="loc_lh_NetRestructuringAndOtherSpecialCharges_9C4E536AEB0F29843F66E764214C370B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_4A1D386163793B76F598E764214CC9DE" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_8B8AB6D0D397C480F0AEE764214CA433" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_4A1D386163793B76F598E764214CC9DE" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/CondensedConsolidatedStatementsOfOperationsUnaudited" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_1385CC2911F451E7A977ED9DAEE9FEE5" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_BC61E121D92B5DCAAED489576552DC20" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_1385CC2911F451E7A977ED9DAEE9FEE5" xlink:to="loc_us-gaap_StatementTable_BC61E121D92B5DCAAED489576552DC20" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_FEB2B814F3D75C488548DE643FEFEA8D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_BC61E121D92B5DCAAED489576552DC20" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_FEB2B814F3D75C488548DE643FEFEA8D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_70FC16C5A6C4BE02261708E13036D72D_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_FEB2B814F3D75C488548DE643FEFEA8D" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_70FC16C5A6C4BE02261708E13036D72D_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9038634FC9B4558FB6710FF321FF08D3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_FEB2B814F3D75C488548DE643FEFEA8D" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9038634FC9B4558FB6710FF321FF08D3" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CovanceMember" xlink:label="loc_lh_CovanceMember_09E5E289AC235C04A2A210063BB5A667" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9038634FC9B4558FB6710FF321FF08D3" xlink:to="loc_lh_CovanceMember_09E5E289AC235C04A2A210063BB5A667" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="loc_us-gaap_StatementScenarioAxis_F445C4A1B7FA59BA86436108F3D80D09" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_BC61E121D92B5DCAAED489576552DC20" xlink:to="loc_us-gaap_StatementScenarioAxis_F445C4A1B7FA59BA86436108F3D80D09" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_691630C3A42D7857E4C908E13036BFA7_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementScenarioAxis_F445C4A1B7FA59BA86436108F3D80D09" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_691630C3A42D7857E4C908E13036BFA7_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_EF2568EF6330534A990514510F3541AA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementScenarioAxis_F445C4A1B7FA59BA86436108F3D80D09" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_EF2568EF6330534A990514510F3541AA" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/Debt" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/DebtConvertibleSubordinatedNotesDetails" xlink:type="extended">
    <loc xlink:href="lh-20160930.xsd#lh_ZeroCouponConvertibleSubordinatedNotesLineItems" xlink:label="loc_lh_ZeroCouponConvertibleSubordinatedNotesLineItems_EB7E3B14DE6E5483ADC36CD57A02F64C" xlink:type="locator" />
    <loc xlink:href="lh-20160930.xsd#lh_ZeroCouponConvertibleSubordinatedNotesTable" xlink:label="loc_lh_ZeroCouponConvertibleSubordinatedNotesTable_4C68B975A1FF532D994FC97F93F29998" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_lh_ZeroCouponConvertibleSubordinatedNotesLineItems_EB7E3B14DE6E5483ADC36CD57A02F64C" xlink:to="loc_lh_ZeroCouponConvertibleSubordinatedNotesTable_4C68B975A1FF532D994FC97F93F29998" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_726B32CEB4735913B667FE70348D7100" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_lh_ZeroCouponConvertibleSubordinatedNotesTable_4C68B975A1FF532D994FC97F93F29998" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_726B32CEB4735913B667FE70348D7100" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_5AA29A463A868B9E0CED325CD6DF0742_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_726B32CEB4735913B667FE70348D7100" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_5AA29A463A868B9E0CED325CD6DF0742_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_A3D8918C7309558BB74C17A3B6915115" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_726B32CEB4735913B667FE70348D7100" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_A3D8918C7309558BB74C17A3B6915115" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SubsequentEventMember" xlink:label="loc_us-gaap_SubsequentEventMember_D586F853170657859713F4B3F8813648" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_A3D8918C7309558BB74C17A3B6915115" xlink:to="loc_us-gaap_SubsequentEventMember_D586F853170657859713F4B3F8813648" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="loc_us-gaap_CreditFacilityAxis_735BEFFB0A3D5C4DB16FA03FFB61BDF7" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_lh_ZeroCouponConvertibleSubordinatedNotesTable_4C68B975A1FF532D994FC97F93F29998" xlink:to="loc_us-gaap_CreditFacilityAxis_735BEFFB0A3D5C4DB16FA03FFB61BDF7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_8EA2D02FD1CEDE6E5324325CD6DF0E30_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_735BEFFB0A3D5C4DB16FA03FFB61BDF7" xlink:to="loc_us-gaap_CreditFacilityDomain_8EA2D02FD1CEDE6E5324325CD6DF0E30_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_DD60982D1A2D5DC296C403E81B86FF83" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_735BEFFB0A3D5C4DB16FA03FFB61BDF7" xlink:to="loc_us-gaap_CreditFacilityDomain_DD60982D1A2D5DC296C403E81B86FF83" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RevolvingCreditFacilityMember" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_8690B73C03FB58A8A9CF81151B99AF8E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_DD60982D1A2D5DC296C403E81B86FF83" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_8690B73C03FB58A8A9CF81151B99AF8E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_E62C5241295D5BC1AA011B7914C790FA" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_lh_ZeroCouponConvertibleSubordinatedNotesTable_4C68B975A1FF532D994FC97F93F29998" xlink:to="loc_us-gaap_RangeAxis_E62C5241295D5BC1AA011B7914C790FA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_C0717F42D7D8C3F80DF0325CD6DF8B4F_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_E62C5241295D5BC1AA011B7914C790FA" xlink:to="loc_us-gaap_RangeMember_C0717F42D7D8C3F80DF0325CD6DF8B4F_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_09DD7F3674D054E796A220DEC16FDD5F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_E62C5241295D5BC1AA011B7914C790FA" xlink:to="loc_us-gaap_RangeMember_09DD7F3674D054E796A220DEC16FDD5F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_FB831CF87FE557BF8F82A8E9F7FE0623" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_09DD7F3674D054E796A220DEC16FDD5F" xlink:to="loc_us-gaap_MaximumMember_FB831CF87FE557BF8F82A8E9F7FE0623" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShortTermDebtTypeAxis" xlink:label="loc_us-gaap_ShortTermDebtTypeAxis_F21760993E3E5A728EB9DEC7F1D810B2" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_lh_ZeroCouponConvertibleSubordinatedNotesTable_4C68B975A1FF532D994FC97F93F29998" xlink:to="loc_us-gaap_ShortTermDebtTypeAxis_F21760993E3E5A728EB9DEC7F1D810B2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShortTermDebtTypeDomain" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_4BBA876C5E140D9F4F20325CD6DFA7B0_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_F21760993E3E5A728EB9DEC7F1D810B2" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_4BBA876C5E140D9F4F20325CD6DFA7B0_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShortTermDebtTypeDomain" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_286D8F07E21C5C3397A0FAAA88F23C88" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_F21760993E3E5A728EB9DEC7F1D810B2" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_286D8F07E21C5C3397A0FAAA88F23C88" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConvertibleSubordinatedDebtMember" xlink:label="loc_us-gaap_ConvertibleSubordinatedDebtMember_0ED69DA0BBE856B9A4375DF277109A64" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_286D8F07E21C5C3397A0FAAA88F23C88" xlink:to="loc_us-gaap_ConvertibleSubordinatedDebtMember_0ED69DA0BBE856B9A4375DF277109A64" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_DebtToEbitdaLeverageRatio" xlink:label="loc_lh_DebtToEbitdaLeverageRatio_6588221CE51C5E16A947861CE11548D3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ZeroCouponConvertibleSubordinatedNotesLineItems_EB7E3B14DE6E5483ADC36CD57A02F64C" xlink:to="loc_lh_DebtToEbitdaLeverageRatio_6588221CE51C5E16A947861CE11548D3" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_DebtInstrumentZeroCouponSubordinatedNotesMinimumContingentCashInterestRate" xlink:label="loc_lh_DebtInstrumentZeroCouponSubordinatedNotesMinimumContingentCashInterestRate_80C8D41821FB5CE4A8CB93FE217E96C1" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ZeroCouponConvertibleSubordinatedNotesLineItems_EB7E3B14DE6E5483ADC36CD57A02F64C" xlink:to="loc_lh_DebtInstrumentZeroCouponSubordinatedNotesMinimumContingentCashInterestRate_80C8D41821FB5CE4A8CB93FE217E96C1" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_NumberOfDaysUsedToEstablishAverageMarketPriceOfZeroCouponSubordinatedNotes" xlink:label="loc_lh_NumberOfDaysUsedToEstablishAverageMarketPriceOfZeroCouponSubordinatedNotes_7905E19F9F6D53B4BCE1352AEA4E6AD8" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ZeroCouponConvertibleSubordinatedNotesLineItems_EB7E3B14DE6E5483ADC36CD57A02F64C" xlink:to="loc_lh_NumberOfDaysUsedToEstablishAverageMarketPriceOfZeroCouponSubordinatedNotes_7905E19F9F6D53B4BCE1352AEA4E6AD8" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_StockConversionRateForZeroCouponSubordinatedNotes" xlink:label="loc_lh_StockConversionRateForZeroCouponSubordinatedNotes_AEA4B049041E582D9DEFCB7F71D71DFC" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ZeroCouponConvertibleSubordinatedNotesLineItems_EB7E3B14DE6E5483ADC36CD57A02F64C" xlink:to="loc_lh_StockConversionRateForZeroCouponSubordinatedNotes_AEA4B049041E582D9DEFCB7F71D71DFC" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Taxbenefitrealizeduponconversionofzerocouponconvertibledebt" xlink:label="loc_lh_Taxbenefitrealizeduponconversionofzerocouponconvertibledebt_CF83549B3D5C5416B6F1615B07F641E8" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ZeroCouponConvertibleSubordinatedNotesLineItems_EB7E3B14DE6E5483ADC36CD57A02F64C" xlink:to="loc_lh_Taxbenefitrealizeduponconversionofzerocouponconvertibledebt_CF83549B3D5C5416B6F1615B07F641E8" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_PrincipalAmountOfZeroCouponSubordinatedNotes" xlink:label="loc_lh_PrincipalAmountOfZeroCouponSubordinatedNotes_E1DB43B1CEE75FD0BB222961961CAB83" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ZeroCouponConvertibleSubordinatedNotesLineItems_EB7E3B14DE6E5483ADC36CD57A02F64C" xlink:to="loc_lh_PrincipalAmountOfZeroCouponSubordinatedNotes_E1DB43B1CEE75FD0BB222961961CAB83" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtConversionConvertedInstrumentAmount1" xlink:label="loc_us-gaap_DebtConversionConvertedInstrumentAmount1_31114D3C5BD25FC7998CDC0F1B9BF5E0" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ZeroCouponConvertibleSubordinatedNotesLineItems_EB7E3B14DE6E5483ADC36CD57A02F64C" xlink:to="loc_us-gaap_DebtConversionConvertedInstrumentAmount1_31114D3C5BD25FC7998CDC0F1B9BF5E0" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_PaymentsOnZeroCouponSubordinatedNotes" xlink:label="loc_lh_PaymentsOnZeroCouponSubordinatedNotes_A4498E717B835403B8EDDE95371FF05B" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ZeroCouponConvertibleSubordinatedNotesLineItems_EB7E3B14DE6E5483ADC36CD57A02F64C" xlink:to="loc_lh_PaymentsOnZeroCouponSubordinatedNotes_A4498E717B835403B8EDDE95371FF05B" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/DebtCreditFacilitiesDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityLineItems" xlink:label="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityTable" xlink:label="loc_us-gaap_LineOfCreditFacilityTable_8A686BFAD2BA631C705307077DD58C09" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_us-gaap_LineOfCreditFacilityTable_8A686BFAD2BA631C705307077DD58C09" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentAxis" xlink:label="loc_us-gaap_DebtInstrumentAxis_BE7F26CA3DFE2034B29A07077DD507BA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_8A686BFAD2BA631C705307077DD58C09" xlink:to="loc_us-gaap_DebtInstrumentAxis_BE7F26CA3DFE2034B29A07077DD507BA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_BE2A122B13F49317F35B07077DD53EEE_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_BE7F26CA3DFE2034B29A07077DD507BA" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_BE2A122B13F49317F35B07077DD53EEE_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_BE2A122B13F49317F35B07077DD53EEE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_BE7F26CA3DFE2034B29A07077DD507BA" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_BE2A122B13F49317F35B07077DD53EEE" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_ExcludingUnderwritingDiscountsOtherFeesMember" xlink:label="loc_lh_ExcludingUnderwritingDiscountsOtherFeesMember_5DA541270DC33BFBCF8007077DD59F5B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_BE2A122B13F49317F35B07077DD53EEE" xlink:to="loc_lh_ExcludingUnderwritingDiscountsOtherFeesMember_5DA541270DC33BFBCF8007077DD59F5B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_VariableRateAxis" xlink:label="loc_us-gaap_VariableRateAxis_BFBA9057B55D12FBCB8307077DD51870" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_8A686BFAD2BA631C705307077DD58C09" xlink:to="loc_us-gaap_VariableRateAxis_BFBA9057B55D12FBCB8307077DD51870" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_VariableRateDomain" xlink:label="loc_us-gaap_VariableRateDomain_D2E9F5E93A3F5EC4CBEA07077DD6452B_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_BFBA9057B55D12FBCB8307077DD51870" xlink:to="loc_us-gaap_VariableRateDomain_D2E9F5E93A3F5EC4CBEA07077DD6452B_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_VariableRateDomain" xlink:label="loc_us-gaap_VariableRateDomain_D2E9F5E93A3F5EC4CBEA07077DD6452B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_BFBA9057B55D12FBCB8307077DD51870" xlink:to="loc_us-gaap_VariableRateDomain_D2E9F5E93A3F5EC4CBEA07077DD6452B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PrimeRateMember" xlink:label="loc_us-gaap_PrimeRateMember_EB50844ADF16936DC9A207077DD6728B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_D2E9F5E93A3F5EC4CBEA07077DD6452B" xlink:to="loc_us-gaap_PrimeRateMember_EB50844ADF16936DC9A207077DD6728B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LondonInterbankOfferedRateLIBORMember" xlink:label="loc_us-gaap_LondonInterbankOfferedRateLIBORMember_6A0CD28DE4C74466DB0E07077DD62B73" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_D2E9F5E93A3F5EC4CBEA07077DD6452B" xlink:to="loc_us-gaap_LondonInterbankOfferedRateLIBORMember_6A0CD28DE4C74466DB0E07077DD62B73" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_5412921DDE9FB59BBE0F07077DD6C321" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_8A686BFAD2BA631C705307077DD58C09" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_5412921DDE9FB59BBE0F07077DD6C321" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_9F3CA508DABE9C7C0C9707077DD6B92E_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_5412921DDE9FB59BBE0F07077DD6C321" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_9F3CA508DABE9C7C0C9707077DD6B92E_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_9F3CA508DABE9C7C0C9707077DD6B92E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_5412921DDE9FB59BBE0F07077DD6C321" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_9F3CA508DABE9C7C0C9707077DD6B92E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SubsequentEventMember" xlink:label="loc_us-gaap_SubsequentEventMember_18CB1FAA40601857041807077DD63C0F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_9F3CA508DABE9C7C0C9707077DD6B92E" xlink:to="loc_us-gaap_SubsequentEventMember_18CB1FAA40601857041807077DD63C0F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_5EDCB2246329552831A607077DD6B604" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_8A686BFAD2BA631C705307077DD58C09" xlink:to="loc_us-gaap_RangeAxis_5EDCB2246329552831A607077DD6B604" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_08DC6ED02DF1A3BA2F6F07077DD6FE54_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_5EDCB2246329552831A607077DD6B604" xlink:to="loc_us-gaap_RangeMember_08DC6ED02DF1A3BA2F6F07077DD6FE54_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_08DC6ED02DF1A3BA2F6F07077DD6FE54" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_5EDCB2246329552831A607077DD6B604" xlink:to="loc_us-gaap_RangeMember_08DC6ED02DF1A3BA2F6F07077DD6FE54" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_41CEF8D3E6701A3DDFE207077DD6A732" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_08DC6ED02DF1A3BA2F6F07077DD6FE54" xlink:to="loc_us-gaap_MaximumMember_41CEF8D3E6701A3DDFE207077DD6A732" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="loc_us-gaap_CreditFacilityAxis_0E338D2F854A66EBFE9107077DD6F51B" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_8A686BFAD2BA631C705307077DD58C09" xlink:to="loc_us-gaap_CreditFacilityAxis_0E338D2F854A66EBFE9107077DD6F51B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_3FB9BB4EB69348D80F9D07077DD60D65_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_0E338D2F854A66EBFE9107077DD6F51B" xlink:to="loc_us-gaap_CreditFacilityDomain_3FB9BB4EB69348D80F9D07077DD60D65_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_3FB9BB4EB69348D80F9D07077DD60D65" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_0E338D2F854A66EBFE9107077DD6F51B" xlink:to="loc_us-gaap_CreditFacilityDomain_3FB9BB4EB69348D80F9D07077DD60D65" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_A60DayDebtBridgeTrauncheDomain" xlink:label="loc_lh_A60DayDebtBridgeTrauncheDomain_17ED0837F4EF176915F107077DD6151C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_3FB9BB4EB69348D80F9D07077DD60D65" xlink:to="loc_lh_A60DayDebtBridgeTrauncheDomain_17ED0837F4EF176915F107077DD6151C" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_TermLoanMember" xlink:label="loc_lh_TermLoanMember_6E8B35630B8F1B3CD5FA07077DD6A616" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_3FB9BB4EB69348D80F9D07077DD60D65" xlink:to="loc_lh_TermLoanMember_6E8B35630B8F1B3CD5FA07077DD6A616" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_TargetRatiotoResumeShareRepurchaseMember" xlink:label="loc_lh_TargetRatiotoResumeShareRepurchaseMember_BFD8101B5D59E7194CDD07077DD6F65A" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_3FB9BB4EB69348D80F9D07077DD60D65" xlink:to="loc_lh_TargetRatiotoResumeShareRepurchaseMember_BFD8101B5D59E7194CDD07077DD6F65A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LondonInterbankOfferedRateLIBORMember" xlink:label="loc_us-gaap_LondonInterbankOfferedRateLIBORMember_3B6703089429FEC38F0D07077DD6547C" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_3FB9BB4EB69348D80F9D07077DD60D65" xlink:to="loc_us-gaap_LondonInterbankOfferedRateLIBORMember_3B6703089429FEC38F0D07077DD6547C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PrimeRateMember" xlink:label="loc_us-gaap_PrimeRateMember_3D5ACC057F849FC7EA2607077DD6A041" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_3FB9BB4EB69348D80F9D07077DD60D65" xlink:to="loc_us-gaap_PrimeRateMember_3D5ACC057F849FC7EA2607077DD6A041" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RevolvingCreditFacilityMember" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_9E1AA032A1F2D172F57F07077DD642B1" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_3FB9BB4EB69348D80F9D07077DD60D65" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_9E1AA032A1F2D172F57F07077DD642B1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShortTermDebtTypeAxis" xlink:label="loc_us-gaap_ShortTermDebtTypeAxis_C1F8228B5CD9C884F12C07077DD72FD2" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_8A686BFAD2BA631C705307077DD58C09" xlink:to="loc_us-gaap_ShortTermDebtTypeAxis_C1F8228B5CD9C884F12C07077DD72FD2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShortTermDebtTypeDomain" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_168F16B0E202D750D93207077DD73F46_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_C1F8228B5CD9C884F12C07077DD72FD2" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_168F16B0E202D750D93207077DD73F46_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShortTermDebtTypeDomain" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_168F16B0E202D750D93207077DD73F46" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_C1F8228B5CD9C884F12C07077DD72FD2" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_168F16B0E202D750D93207077DD73F46" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RevolvingCreditFacilityMember" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_FAF99498AF0EACFBE0D407077DD770F7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_168F16B0E202D750D93207077DD73F46" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_FAF99498AF0EACFBE0D407077DD770F7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_C90579AFFBFD0219C18407077DD77EDD" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_8A686BFAD2BA631C705307077DD58C09" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_C90579AFFBFD0219C18407077DD77EDD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_8AE88D72C8546FBA304907077DD7C707_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_C90579AFFBFD0219C18407077DD77EDD" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_8AE88D72C8546FBA304907077DD7C707_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_8AE88D72C8546FBA304907077DD7C707" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_C90579AFFBFD0219C18407077DD77EDD" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_8AE88D72C8546FBA304907077DD7C707" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_SeniorNotesDue2020Member" xlink:label="loc_lh_SeniorNotesDue2020Member_AAC61A28EF5CA50934AB07077DD795B7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_8AE88D72C8546FBA304907077DD7C707" xlink:to="loc_lh_SeniorNotesDue2020Member_AAC61A28EF5CA50934AB07077DD795B7" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Seniornotesdue2025Member" xlink:label="loc_lh_Seniornotesdue2025Member_CE92BF2B1D0A8B33944607077DD706ED" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_8AE88D72C8546FBA304907077DD7C707" xlink:to="loc_lh_Seniornotesdue2025Member_CE92BF2B1D0A8B33944607077DD706ED" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Seniornotesdue2045Member" xlink:label="loc_lh_Seniornotesdue2045Member_71E2E7F74AD1A4C99A2D07077DD7BED7" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_8AE88D72C8546FBA304907077DD7C707" xlink:to="loc_lh_Seniornotesdue2045Member_71E2E7F74AD1A4C99A2D07077DD7BED7" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_SeniorNotesDue2022Member" xlink:label="loc_lh_SeniorNotesDue2022Member_340C881CB8799D95B48007077DD7304E" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_8AE88D72C8546FBA304907077DD7C707" xlink:to="loc_lh_SeniorNotesDue2022Member_340C881CB8799D95B48007077DD7304E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="loc_us-gaap_StatementScenarioAxis_E87AD5811F572D4C244E07077DD71FC0" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_8A686BFAD2BA631C705307077DD58C09" xlink:to="loc_us-gaap_StatementScenarioAxis_E87AD5811F572D4C244E07077DD71FC0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_80C5CAC141F99B2C068307077DD73417_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementScenarioAxis_E87AD5811F572D4C244E07077DD71FC0" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_80C5CAC141F99B2C068307077DD73417_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_80C5CAC141F99B2C068307077DD73417" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementScenarioAxis_E87AD5811F572D4C244E07077DD71FC0" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_80C5CAC141F99B2C068307077DD73417" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityAxis" xlink:label="loc_us-gaap_LineOfCreditFacilityAxis_5F6556DFC17512C60C4C07077DD7D215" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_8A686BFAD2BA631C705307077DD58C09" xlink:to="loc_us-gaap_LineOfCreditFacilityAxis_5F6556DFC17512C60C4C07077DD7D215" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityLenderDomain" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_7116E1294FB478F4DBDB07077DD7F3B4_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LineOfCreditFacilityAxis_5F6556DFC17512C60C4C07077DD7D215" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_7116E1294FB478F4DBDB07077DD7F3B4_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityLenderDomain" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_7116E1294FB478F4DBDB07077DD7F3B4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LineOfCreditFacilityAxis_5F6556DFC17512C60C4C07077DD7D215" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_7116E1294FB478F4DBDB07077DD7F3B4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongTermDebtMember" xlink:label="loc_us-gaap_LongTermDebtMember_BD7D3E91ACFB0425CE2307077DD796E9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLenderDomain_7116E1294FB478F4DBDB07077DD7F3B4" xlink:to="loc_us-gaap_LongTermDebtMember_BD7D3E91ACFB0425CE2307077DD796E9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RevolvingCreditFacilityMember" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_EA9A387FEA134BBF90A707077DD791CD" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLenderDomain_7116E1294FB478F4DBDB07077DD7F3B4" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_EA9A387FEA134BBF90A707077DD791CD" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_PreviouslyRecordedLitigationReserveInConnectionWithFalseClaimsActLawsuit" xlink:label="loc_lh_PreviouslyRecordedLitigationReserveInConnectionWithFalseClaimsActLawsuit_01EB555CC737B8C1C3C307077DD7724C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_lh_PreviouslyRecordedLitigationReserveInConnectionWithFalseClaimsActLawsuit_01EB555CC737B8C1C3C307077DD7724C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SeniorLongTermNotes" xlink:label="loc_us-gaap_SeniorLongTermNotes_184C11AAE61EADCDB57F07077DD7C5B4" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_us-gaap_SeniorLongTermNotes_184C11AAE61EADCDB57F07077DD7C5B4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_6DAA5C977AD761FF49FF07077DD894D8" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_6DAA5C977AD761FF49FF07077DD894D8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConvertibleSubordinatedDebtCurrent" xlink:label="loc_us-gaap_ConvertibleSubordinatedDebtCurrent_E05A9118758D21D0D30007077DD8F6D8" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_us-gaap_ConvertibleSubordinatedDebtCurrent_E05A9118758D21D0D30007077DD8F6D8" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_SeniorNotescurrent2015" xlink:label="loc_lh_SeniorNotescurrent2015_E8AB1B1E910B6168252507077DD82510" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_lh_SeniorNotescurrent2015_E8AB1B1E910B6168252507077DD82510" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongTermDebtMaturitiesRepaymentTerms" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentTerms_46C83DA0F7DE09D82C0B07077DD8FFC3" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentTerms_46C83DA0F7DE09D82C0B07077DD8FFC3" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CreditFacilityMaximumLettersofCredit" xlink:label="loc_lh_CreditFacilityMaximumLettersofCredit_2207E1580D4A604E90B307077DD81B0A" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_lh_CreditFacilityMaximumLettersofCredit_2207E1580D4A604E90B307077DD81B0A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LinesOfCreditCurrent" xlink:label="loc_us-gaap_LinesOfCreditCurrent_C90E3B498BC3D29273E207077DD8129C" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_us-gaap_LinesOfCreditCurrent_C90E3B498BC3D29273E207077DD8129C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding" xlink:label="loc_us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding_F83E2A4AA596295F695D07077DD8B55E" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding_F83E2A4AA596295F695D07077DD8B55E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_3BD84F57D8EA6487ABCC07077DD8C8EB" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_3BD84F57D8EA6487ABCC07077DD8C8EB" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_DebtToEbitdaLeverageRatio" xlink:label="loc_lh_DebtToEbitdaLeverageRatio_28FA45132CA2C3F542BF07077DD8CD38" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_lh_DebtToEbitdaLeverageRatio_28FA45132CA2C3F542BF07077DD8CD38" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_2E3BB9EC4FA005F5162D07077DD88EB7" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_2E3BB9EC4FA005F5162D07077DD88EB7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityInterestRateDescription" xlink:label="loc_us-gaap_LineOfCreditFacilityInterestRateDescription_A76B07FE9A750C48BC2B07077DD8315A" xlink:type="locator" />
    <definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_us-gaap_LineOfCreditFacilityInterestRateDescription_A76B07FE9A750C48BC2B07077DD8315A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CapitalLeaseObligationsCurrent" xlink:label="loc_us-gaap_CapitalLeaseObligationsCurrent_19F208DB1005F38E955607077DD8775F" xlink:type="locator" />
    <definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_us-gaap_CapitalLeaseObligationsCurrent_19F208DB1005F38E955607077DD8775F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtCurrent" xlink:label="loc_us-gaap_DebtCurrent_34794B2D5B732A824DCA07077DD95631" xlink:type="locator" />
    <definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_us-gaap_DebtCurrent_34794B2D5B732A824DCA07077DD95631" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_OtherLongTermDebt" xlink:label="loc_us-gaap_OtherLongTermDebt_8D00C63A19E36C44E58A07077DD98BEF" xlink:type="locator" />
    <definitionArc order="16" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_us-gaap_OtherLongTermDebt_8D00C63A19E36C44E58A07077DD98BEF" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_BridgeTermCreditFacilityAgreementMaximumBorrowingAmount" xlink:label="loc_lh_BridgeTermCreditFacilityAgreementMaximumBorrowingAmount_A6CEB5A093BDC05544A707077DD95266" xlink:type="locator" />
    <definitionArc order="17" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_lh_BridgeTermCreditFacilityAgreementMaximumBorrowingAmount_A6CEB5A093BDC05544A707077DD95266" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CreditFacilityOptiontoIncrease" xlink:label="loc_lh_CreditFacilityOptiontoIncrease_953F1F325E65518087D807077DD942A6" xlink:type="locator" />
    <definitionArc order="18" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_lh_CreditFacilityOptiontoIncrease_953F1F325E65518087D807077DD942A6" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CreditFacilityMaximumSwingLineBorrowings" xlink:label="loc_lh_CreditFacilityMaximumSwingLineBorrowings_FDFBC43DEA80CA4F7C0607077DD96BAA" xlink:type="locator" />
    <definitionArc order="19" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_lh_CreditFacilityMaximumSwingLineBorrowings_FDFBC43DEA80CA4F7C0607077DD96BAA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityCommitmentFeeDescription" xlink:label="loc_us-gaap_LineOfCreditFacilityCommitmentFeeDescription_38AFFDA990464500051D07077DD9D128" xlink:type="locator" />
    <definitionArc order="20" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_us-gaap_LineOfCreditFacilityCommitmentFeeDescription_38AFFDA990464500051D07077DD9D128" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd" xlink:label="loc_us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd_2A2B41BA9E04B087383107077DD9D84B" xlink:type="locator" />
    <definitionArc order="21" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_FEFFD5EEDD0CEB7ECC7407077DD7547D" xlink:to="loc_us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd_2A2B41BA9E04B087383107077DD9D84B" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/DebtLongTermDebtTableDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="loc_us-gaap_DebtInstrumentLineItems_A8BCEE53284F96BEBE3EE76420C5EA4B" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentTable" xlink:label="loc_us-gaap_DebtInstrumentTable_6DA1F5F7781A5D1A2ACBE76420C3A0EB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A8BCEE53284F96BEBE3EE76420C5EA4B" xlink:to="loc_us-gaap_DebtInstrumentTable_6DA1F5F7781A5D1A2ACBE76420C3A0EB" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="loc_us-gaap_StatementScenarioAxis_D4CDC8B5D2FE60B2C909E76420C38C5E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_6DA1F5F7781A5D1A2ACBE76420C3A0EB" xlink:to="loc_us-gaap_StatementScenarioAxis_D4CDC8B5D2FE60B2C909E76420C38C5E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_E9843EF1E9620688E2B1E76420C35668_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementScenarioAxis_D4CDC8B5D2FE60B2C909E76420C38C5E" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_E9843EF1E9620688E2B1E76420C35668_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_E9843EF1E9620688E2B1E76420C35668" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementScenarioAxis_D4CDC8B5D2FE60B2C909E76420C38C5E" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_E9843EF1E9620688E2B1E76420C35668" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_VariableRateAxis" xlink:label="loc_us-gaap_VariableRateAxis_3CFA184F500E64DA8F62E76420C3CDA5" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_6DA1F5F7781A5D1A2ACBE76420C3A0EB" xlink:to="loc_us-gaap_VariableRateAxis_3CFA184F500E64DA8F62E76420C3CDA5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_VariableRateDomain" xlink:label="loc_us-gaap_VariableRateDomain_018D17CD527FF6602EBFE76420C3DF37_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_3CFA184F500E64DA8F62E76420C3CDA5" xlink:to="loc_us-gaap_VariableRateDomain_018D17CD527FF6602EBFE76420C3DF37_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_VariableRateDomain" xlink:label="loc_us-gaap_VariableRateDomain_018D17CD527FF6602EBFE76420C3DF37" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_3CFA184F500E64DA8F62E76420C3CDA5" xlink:to="loc_us-gaap_VariableRateDomain_018D17CD527FF6602EBFE76420C3DF37" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PrimeRateMember" xlink:label="loc_us-gaap_PrimeRateMember_275907FCA56C21499445E76420C3EA4C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_018D17CD527FF6602EBFE76420C3DF37" xlink:to="loc_us-gaap_PrimeRateMember_275907FCA56C21499445E76420C3EA4C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LondonInterbankOfferedRateLIBORMember" xlink:label="loc_us-gaap_LondonInterbankOfferedRateLIBORMember_41A203A31D43DE3E0239E76420C3A341" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PrimeRateMember_275907FCA56C21499445E76420C3EA4C" xlink:to="loc_us-gaap_LondonInterbankOfferedRateLIBORMember_41A203A31D43DE3E0239E76420C3A341" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_EAC390C9E203B9D2C87FE76420C42465" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_6DA1F5F7781A5D1A2ACBE76420C3A0EB" xlink:to="loc_us-gaap_RangeAxis_EAC390C9E203B9D2C87FE76420C42465" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_683C0E8F510883E1F29EE76420C45F72_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_EAC390C9E203B9D2C87FE76420C42465" xlink:to="loc_us-gaap_RangeMember_683C0E8F510883E1F29EE76420C45F72_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_683C0E8F510883E1F29EE76420C45F72" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_EAC390C9E203B9D2C87FE76420C42465" xlink:to="loc_us-gaap_RangeMember_683C0E8F510883E1F29EE76420C45F72" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_ADF67F4F37B1CA51B950E76420C4DB3A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_683C0E8F510883E1F29EE76420C45F72" xlink:to="loc_us-gaap_MaximumMember_ADF67F4F37B1CA51B950E76420C4DB3A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_0325362A203AAD78B60CE76420C4346F" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_6DA1F5F7781A5D1A2ACBE76420C3A0EB" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_0325362A203AAD78B60CE76420C4346F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_E5E03F1ABC19C3566046E76420C4E252_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_0325362A203AAD78B60CE76420C4346F" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_E5E03F1ABC19C3566046E76420C4E252_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_E5E03F1ABC19C3566046E76420C4E252" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_0325362A203AAD78B60CE76420C4346F" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_E5E03F1ABC19C3566046E76420C4E252" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="loc_us-gaap_CreditFacilityAxis_9ACC1056CDC2E03A509EE76420C4E92D" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_6DA1F5F7781A5D1A2ACBE76420C3A0EB" xlink:to="loc_us-gaap_CreditFacilityAxis_9ACC1056CDC2E03A509EE76420C4E92D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_A55480171825143831EDE76420C445F3_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_9ACC1056CDC2E03A509EE76420C4E92D" xlink:to="loc_us-gaap_CreditFacilityDomain_A55480171825143831EDE76420C445F3_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_A55480171825143831EDE76420C445F3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_9ACC1056CDC2E03A509EE76420C4E92D" xlink:to="loc_us-gaap_CreditFacilityDomain_A55480171825143831EDE76420C445F3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_CBC6B85ED9BDF152DD82E76420C48A9E" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_6DA1F5F7781A5D1A2ACBE76420C3A0EB" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_CBC6B85ED9BDF152DD82E76420C48A9E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_39781C50B8AB6FEE83B7E76420C43274_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_CBC6B85ED9BDF152DD82E76420C48A9E" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_39781C50B8AB6FEE83B7E76420C43274_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_39781C50B8AB6FEE83B7E76420C43274" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_CBC6B85ED9BDF152DD82E76420C48A9E" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_39781C50B8AB6FEE83B7E76420C43274" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CovanceMember" xlink:label="loc_lh_CovanceMember_DE45DF82F466BBD8FFAAE76420C4876F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_39781C50B8AB6FEE83B7E76420C43274" xlink:to="loc_lh_CovanceMember_DE45DF82F466BBD8FFAAE76420C4876F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityAxis" xlink:label="loc_us-gaap_LineOfCreditFacilityAxis_727F0C1B15FBA8657270E76420C4ED77" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_6DA1F5F7781A5D1A2ACBE76420C3A0EB" xlink:to="loc_us-gaap_LineOfCreditFacilityAxis_727F0C1B15FBA8657270E76420C4ED77" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityLenderDomain" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_678B5D49148E291FE2AAE76420C49E41_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LineOfCreditFacilityAxis_727F0C1B15FBA8657270E76420C4ED77" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_678B5D49148E291FE2AAE76420C49E41_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityLenderDomain" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_678B5D49148E291FE2AAE76420C49E41" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LineOfCreditFacilityAxis_727F0C1B15FBA8657270E76420C4ED77" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_678B5D49148E291FE2AAE76420C49E41" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongTermDebtMember" xlink:label="loc_us-gaap_LongTermDebtMember_D53A9D6CAFCEDA82DF30E76420C4B81A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLenderDomain_678B5D49148E291FE2AAE76420C49E41" xlink:to="loc_us-gaap_LongTermDebtMember_D53A9D6CAFCEDA82DF30E76420C4B81A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_DEEF6E68C7EBF13FC699E76420C41ED1" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_6DA1F5F7781A5D1A2ACBE76420C3A0EB" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_DEEF6E68C7EBF13FC699E76420C41ED1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_CCE7E25ECDB17FADE686E76420C4E6D2_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_DEEF6E68C7EBF13FC699E76420C41ED1" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_CCE7E25ECDB17FADE686E76420C4E6D2_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_CCE7E25ECDB17FADE686E76420C4E6D2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_DEEF6E68C7EBF13FC699E76420C41ED1" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_CCE7E25ECDB17FADE686E76420C4E6D2" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_SeniorNotesDue2017Member" xlink:label="loc_lh_SeniorNotesDue2017Member_C41BC3D4578EFEC5882CE76420C43960" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_CCE7E25ECDB17FADE686E76420C4E6D2" xlink:to="loc_lh_SeniorNotesDue2017Member_C41BC3D4578EFEC5882CE76420C43960" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Seniornotesdue2018Member" xlink:label="loc_lh_Seniornotesdue2018Member_CDB39F28D4201421AAD1E76420C58EE6" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_CCE7E25ECDB17FADE686E76420C4E6D2" xlink:to="loc_lh_Seniornotesdue2018Member_CDB39F28D4201421AAD1E76420C58EE6" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_SeniorNotesDue2020Member" xlink:label="loc_lh_SeniorNotesDue2020Member_9CC7503796FA592285C3E76420C57C70" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_CCE7E25ECDB17FADE686E76420C4E6D2" xlink:to="loc_lh_SeniorNotesDue2020Member_9CC7503796FA592285C3E76420C57C70" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_SeniorNotesDue2022Member" xlink:label="loc_lh_SeniorNotesDue2022Member_104D9317875A023E75D1E76420C54F2E" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_CCE7E25ECDB17FADE686E76420C4E6D2" xlink:to="loc_lh_SeniorNotesDue2022Member_104D9317875A023E75D1E76420C54F2E" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Seniornotesdue2025Member" xlink:label="loc_lh_Seniornotesdue2025Member_8359B7838C20E76BD5D8E76420C55166" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_CCE7E25ECDB17FADE686E76420C4E6D2" xlink:to="loc_lh_Seniornotesdue2025Member_8359B7838C20E76BD5D8E76420C55166" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Seniornotesdue2023Member" xlink:label="loc_lh_Seniornotesdue2023Member_4350AE81C51286CF1544E76420C5E5A1" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_CCE7E25ECDB17FADE686E76420C4E6D2" xlink:to="loc_lh_Seniornotesdue2023Member_4350AE81C51286CF1544E76420C5E5A1" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Seniornotesdue2025MemberMember" xlink:label="loc_lh_Seniornotesdue2025MemberMember_CD90D172B75265357124E76420C5DFAD" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_CCE7E25ECDB17FADE686E76420C4E6D2" xlink:to="loc_lh_Seniornotesdue2025MemberMember_CD90D172B75265357124E76420C5DFAD" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Seniornotesdue2045Member" xlink:label="loc_lh_Seniornotesdue2045Member_639981038CC5ED7E3A64E76420C5011C" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_CCE7E25ECDB17FADE686E76420C4E6D2" xlink:to="loc_lh_Seniornotesdue2045Member_639981038CC5ED7E3A64E76420C5011C" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_BridgeTermCreditFacilityAgreementMaximumBorrowingAmount" xlink:label="loc_lh_BridgeTermCreditFacilityAgreementMaximumBorrowingAmount_6302158E0FA400BAD8B0E76420C570F0" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A8BCEE53284F96BEBE3EE76420C5EA4B" xlink:to="loc_lh_BridgeTermCreditFacilityAgreementMaximumBorrowingAmount_6302158E0FA400BAD8B0E76420C570F0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_231B70C6C602525E94D0E76420C5CBC9" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A8BCEE53284F96BEBE3EE76420C5EA4B" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_231B70C6C602525E94D0E76420C5CBC9" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_DebtToEbitdaLeverageRatio" xlink:label="loc_lh_DebtToEbitdaLeverageRatio_887FF09DAA932C3EA97EE76420C5C0AB" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A8BCEE53284F96BEBE3EE76420C5EA4B" xlink:to="loc_lh_DebtToEbitdaLeverageRatio_887FF09DAA932C3EA97EE76420C5C0AB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityInterestRateDescription" xlink:label="loc_us-gaap_LineOfCreditFacilityInterestRateDescription_81A95490BB4CA41F784CE76420C530D2" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A8BCEE53284F96BEBE3EE76420C5EA4B" xlink:to="loc_us-gaap_LineOfCreditFacilityInterestRateDescription_81A95490BB4CA41F784CE76420C530D2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityCommitmentFeeDescription" xlink:label="loc_us-gaap_LineOfCreditFacilityCommitmentFeeDescription_E304A3E5FC76C0BB5100E76420C5DF58" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A8BCEE53284F96BEBE3EE76420C5EA4B" xlink:to="loc_us-gaap_LineOfCreditFacilityCommitmentFeeDescription_E304A3E5FC76C0BB5100E76420C5DF58" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongTermDebtNoncurrent" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_45ABA1FABA32C3C7476EE76420C5583A" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A8BCEE53284F96BEBE3EE76420C5EA4B" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_45ABA1FABA32C3C7476EE76420C5583A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding" xlink:label="loc_us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding_36972D9AD7053C0E2F4FE76420C53C1C" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A8BCEE53284F96BEBE3EE76420C5EA4B" xlink:to="loc_us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding_36972D9AD7053C0E2F4FE76420C53C1C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_OtherLongTermDebt" xlink:label="loc_us-gaap_OtherLongTermDebt_22379576C4166287D948E76420C5FB58" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A8BCEE53284F96BEBE3EE76420C5EA4B" xlink:to="loc_us-gaap_OtherLongTermDebt_22379576C4166287D948E76420C5FB58" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SeniorLongTermNotes" xlink:label="loc_us-gaap_SeniorLongTermNotes_05F52B718E87D184970EE76420C59294" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A8BCEE53284F96BEBE3EE76420C5EA4B" xlink:to="loc_us-gaap_SeniorLongTermNotes_05F52B718E87D184970EE76420C59294" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CapitalLeaseObligationsNoncurrent" xlink:label="loc_us-gaap_CapitalLeaseObligationsNoncurrent_DD466D40471EBCC3DAEDE76420C5FE60" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A8BCEE53284F96BEBE3EE76420C5EA4B" xlink:to="loc_us-gaap_CapitalLeaseObligationsNoncurrent_DD466D40471EBCC3DAEDE76420C5FE60" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_433592CCBB0E1296BB58E76420C5D28A" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A8BCEE53284F96BEBE3EE76420C5EA4B" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_433592CCBB0E1296BB58E76420C5D28A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongTermDebtMaturitiesRepaymentTerms" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentTerms_CEA7FDB0753D705CD34FE76420C51FFA" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A8BCEE53284F96BEBE3EE76420C5EA4B" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentTerms_CEA7FDB0753D705CD34FE76420C51FFA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_F4603F16D0EEC5499140E76420C6BAB6" xlink:type="locator" />
    <definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A8BCEE53284F96BEBE3EE76420C5EA4B" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_F4603F16D0EEC5499140E76420C6BAB6" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CreditFacilityOptiontoIncrease" xlink:label="loc_lh_CreditFacilityOptiontoIncrease_6015781B2465C0C80BC5E76420C69B6D" xlink:type="locator" />
    <definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A8BCEE53284F96BEBE3EE76420C5EA4B" xlink:to="loc_lh_CreditFacilityOptiontoIncrease_6015781B2465C0C80BC5E76420C69B6D" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CreditFacilityMaximumSwingLineBorrowings" xlink:label="loc_lh_CreditFacilityMaximumSwingLineBorrowings_17CDDA6656349A8E126BE76420C603C0" xlink:type="locator" />
    <definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A8BCEE53284F96BEBE3EE76420C5EA4B" xlink:to="loc_lh_CreditFacilityMaximumSwingLineBorrowings_17CDDA6656349A8E126BE76420C603C0" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CreditFacilityMaximumLettersofCredit" xlink:label="loc_lh_CreditFacilityMaximumLettersofCredit_BB0E01CA8A9BFA7DF875E76420C6C5B1" xlink:type="locator" />
    <definitionArc order="16" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A8BCEE53284F96BEBE3EE76420C5EA4B" xlink:to="loc_lh_CreditFacilityMaximumLettersofCredit_BB0E01CA8A9BFA7DF875E76420C6C5B1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfDebtInstrumentsTextBlock" xlink:label="loc_us-gaap_ScheduleOfDebtInstrumentsTextBlock_6023566928FCFAF1D260E76420C603C6" xlink:type="locator" />
    <definitionArc order="17" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_A8BCEE53284F96BEBE3EE76420C5EA4B" xlink:to="loc_us-gaap_ScheduleOfDebtInstrumentsTextBlock_6023566928FCFAF1D260E76420C603C6" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/DebtSeniorNotesDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentTable" xlink:label="loc_us-gaap_DebtInstrumentTable_57F2D1E48F45C7519DAE079421FF9FE6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_us-gaap_DebtInstrumentTable_57F2D1E48F45C7519DAE079421FF9FE6" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShortTermDebtTypeAxis" xlink:label="loc_us-gaap_ShortTermDebtTypeAxis_02F0E5ABBE9C9989A2AB079421FFA9BD" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_57F2D1E48F45C7519DAE079421FF9FE6" xlink:to="loc_us-gaap_ShortTermDebtTypeAxis_02F0E5ABBE9C9989A2AB079421FFA9BD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShortTermDebtTypeDomain" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_47DAF91F671E25782D6D079421FF33C5_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_02F0E5ABBE9C9989A2AB079421FFA9BD" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_47DAF91F671E25782D6D079421FF33C5_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShortTermDebtTypeDomain" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_47DAF91F671E25782D6D079421FF33C5" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_02F0E5ABBE9C9989A2AB079421FFA9BD" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_47DAF91F671E25782D6D079421FF33C5" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Seniornotesdue2018Member" xlink:label="loc_lh_Seniornotesdue2018Member_50748EC9E5A03555FE160794220065C8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_47DAF91F671E25782D6D079421FF33C5" xlink:to="loc_lh_Seniornotesdue2018Member_50748EC9E5A03555FE160794220065C8" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_SeniorNotesDue2017Member" xlink:label="loc_lh_SeniorNotesDue2017Member_1622B8930FE16621E9D5079422000418" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_47DAF91F671E25782D6D079421FF33C5" xlink:to="loc_lh_SeniorNotesDue2017Member_1622B8930FE16621E9D5079422000418" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_D1AE2D0C486D695DF0DD079422001289" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_57F2D1E48F45C7519DAE079421FF9FE6" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_D1AE2D0C486D695DF0DD079422001289" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_571C1CB339430BA806F4079422006812_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_D1AE2D0C486D695DF0DD079422001289" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_571C1CB339430BA806F4079422006812_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_571C1CB339430BA806F4079422006812" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_D1AE2D0C486D695DF0DD079422001289" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_571C1CB339430BA806F4079422006812" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CovanceMember" xlink:label="loc_lh_CovanceMember_95F4C6203389C39D2B89079422003865" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_571C1CB339430BA806F4079422006812" xlink:to="loc_lh_CovanceMember_95F4C6203389C39D2B89079422003865" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentAxis" xlink:label="loc_us-gaap_DebtInstrumentAxis_2B1CA056C73E015EA18607942200A47B" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_57F2D1E48F45C7519DAE079421FF9FE6" xlink:to="loc_us-gaap_DebtInstrumentAxis_2B1CA056C73E015EA18607942200A47B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_7FC8D3D6681183E95D5C0794220053D9_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_2B1CA056C73E015EA18607942200A47B" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_7FC8D3D6681183E95D5C0794220053D9_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_7FC8D3D6681183E95D5C0794220053D9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_2B1CA056C73E015EA18607942200A47B" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_7FC8D3D6681183E95D5C0794220053D9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_04D3CD8CB00E34EEAA76079422000795" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_57F2D1E48F45C7519DAE079421FF9FE6" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_04D3CD8CB00E34EEAA76079422000795" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_D1CB37CF2BF55495A9950794220046DE_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_04D3CD8CB00E34EEAA76079422000795" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_D1CB37CF2BF55495A9950794220046DE_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_D1CB37CF2BF55495A9950794220046DE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_04D3CD8CB00E34EEAA76079422000795" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_D1CB37CF2BF55495A9950794220046DE" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Seniornotesdue2018Member" xlink:label="loc_lh_Seniornotesdue2018Member_AE179AA7129DA78C641707942200F748" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_D1CB37CF2BF55495A9950794220046DE" xlink:to="loc_lh_Seniornotesdue2018Member_AE179AA7129DA78C641707942200F748" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Seniornotesdue2023Member" xlink:label="loc_lh_Seniornotesdue2023Member_AEF87B8CEAFC9437D8D30794220054C5" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_D1CB37CF2BF55495A9950794220046DE" xlink:to="loc_lh_Seniornotesdue2023Member_AEF87B8CEAFC9437D8D30794220054C5" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Seniornotesdue2025MemberMember" xlink:label="loc_lh_Seniornotesdue2025MemberMember_220AB0691ECF3E61E00207942200112C" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_D1CB37CF2BF55495A9950794220046DE" xlink:to="loc_lh_Seniornotesdue2025MemberMember_220AB0691ECF3E61E00207942200112C" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Seniornotesdue2045Member" xlink:label="loc_lh_Seniornotesdue2045Member_F867030378C7A8846317079422004FC1" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_D1CB37CF2BF55495A9950794220046DE" xlink:to="loc_lh_Seniornotesdue2045Member_F867030378C7A8846317079422004FC1" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_SeniorNotesDue2022Member" xlink:label="loc_lh_SeniorNotesDue2022Member_F152B2E4E55527380AE0079422004413" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_D1CB37CF2BF55495A9950794220046DE" xlink:to="loc_lh_SeniorNotesDue2022Member_F152B2E4E55527380AE0079422004413" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Seniornotesdue2025Member" xlink:label="loc_lh_Seniornotesdue2025Member_5D5D1FD716CC4DFD6EBE079422008D66" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_D1CB37CF2BF55495A9950794220046DE" xlink:to="loc_lh_Seniornotesdue2025Member_5D5D1FD716CC4DFD6EBE079422008D66" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_SeniorNotesDue2017Member" xlink:label="loc_lh_SeniorNotesDue2017Member_05873420145FE42AF3E607942200AC97" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_D1CB37CF2BF55495A9950794220046DE" xlink:to="loc_lh_SeniorNotesDue2017Member_05873420145FE42AF3E607942200AC97" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_SeniorNotesDue2020Member" xlink:label="loc_lh_SeniorNotesDue2020Member_7C271300514C8E240AA207942201D3B8" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_D1CB37CF2BF55495A9950794220046DE" xlink:to="loc_lh_SeniorNotesDue2020Member_7C271300514C8E240AA207942201D3B8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LinesOfCreditCurrent" xlink:label="loc_us-gaap_LinesOfCreditCurrent_9A2A8D219AAA9262EAB5079422010B5A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_us-gaap_LinesOfCreditCurrent_9A2A8D219AAA9262EAB5079422010B5A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConvertibleSubordinatedDebtCurrent" xlink:label="loc_us-gaap_ConvertibleSubordinatedDebtCurrent_9B93C2C7429B9A993C36079422019A3C" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_us-gaap_ConvertibleSubordinatedDebtCurrent_9B93C2C7429B9A993C36079422019A3C" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_SeniorNotescurrent2015" xlink:label="loc_lh_SeniorNotescurrent2015_98BD31B3F0B79147F81B079422018DD4" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_lh_SeniorNotescurrent2015_98BD31B3F0B79147F81B079422018DD4" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Shorttermdebtissuancecosts" xlink:label="loc_lh_Shorttermdebtissuancecosts_1FBD6FACE875A79FBE8107942201D7D3" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_lh_Shorttermdebtissuancecosts_1FBD6FACE875A79FBE8107942201D7D3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongTermDebtNoncurrent" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_6D2AB1546ACD2B9E4593079422011891" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_6D2AB1546ACD2B9E4593079422011891" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Sequenomnotesretired" xlink:label="loc_lh_Sequenomnotesretired_4E666B22F8BF66CF6BEC07981EB9F016" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_lh_Sequenomnotesretired_4E666B22F8BF66CF6BEC07981EB9F016" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesLongTermDebt" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesLongTermDebt_0732CDB0F45A282BACAF07942201FF5F" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesLongTermDebt_0732CDB0F45A282BACAF07942201FF5F" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Makewholepayment" xlink:label="loc_lh_Makewholepayment_EAE711B0F59D39AB56AF07942201A166" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_lh_Makewholepayment_EAE711B0F59D39AB56AF07942201A166" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_AmortizationOfDebtDiscountPremium" xlink:label="loc_us-gaap_AmortizationOfDebtDiscountPremium_82B19F70E3358DCBB6CE07942201B3EA" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_us-gaap_AmortizationOfDebtDiscountPremium_82B19F70E3358DCBB6CE07942201B3EA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_9F0BEC15ABA55B8DB3B00794220177A3" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_9F0BEC15ABA55B8DB3B00794220177A3" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_BridgeTermCreditFacilityAgreementMaximumBorrowingAmount" xlink:label="loc_lh_BridgeTermCreditFacilityAgreementMaximumBorrowingAmount_9D048A3367496A46584F07942201C0AF" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_lh_BridgeTermCreditFacilityAgreementMaximumBorrowingAmount_9D048A3367496A46584F07942201C0AF" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CreditFacilityOptiontoIncrease" xlink:label="loc_lh_CreditFacilityOptiontoIncrease_C39E302FF0732FF6BE7807942201A658" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_lh_CreditFacilityOptiontoIncrease_C39E302FF0732FF6BE7807942201A658" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CreditFacilityMaximumSwingLineBorrowings" xlink:label="loc_lh_CreditFacilityMaximumSwingLineBorrowings_4ABB79C8171CBB34310A079422018C03" xlink:type="locator" />
    <definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_lh_CreditFacilityMaximumSwingLineBorrowings_4ABB79C8171CBB34310A079422018C03" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CreditFacilityMaximumLettersofCredit" xlink:label="loc_lh_CreditFacilityMaximumLettersofCredit_AD82797BD0CCEBEC71C607942201852E" xlink:type="locator" />
    <definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_lh_CreditFacilityMaximumLettersofCredit_AD82797BD0CCEBEC71C607942201852E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SeniorLongTermNotes" xlink:label="loc_us-gaap_SeniorLongTermNotes_4283891D45A7338A7B3A079422011119" xlink:type="locator" />
    <definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_us-gaap_SeniorLongTermNotes_4283891D45A7338A7B3A079422011119" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_DebtToEbitdaLeverageRatio" xlink:label="loc_lh_DebtToEbitdaLeverageRatio_970B5F34064A8A6C0F1707942201027F" xlink:type="locator" />
    <definitionArc order="16" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_lh_DebtToEbitdaLeverageRatio_970B5F34064A8A6C0F1707942201027F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueHedgesAtFairValueNet" xlink:label="loc_us-gaap_FairValueHedgesAtFairValueNet_1FBE7D48F8FA5F35C46C07942201B197" xlink:type="locator" />
    <definitionArc order="17" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_us-gaap_FairValueHedgesAtFairValueNet_1FBE7D48F8FA5F35C46C07942201B197" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_4C9AC7D67A208C314E2B079422013FCD" xlink:type="locator" />
    <definitionArc order="18" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_4C9AC7D67A208C314E2B079422013FCD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_A18B17007EF1C14C0CF707942201A489" xlink:type="locator" />
    <definitionArc order="19" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_A18B17007EF1C14C0CF707942201A489" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityInterestRateDescription" xlink:label="loc_us-gaap_LineOfCreditFacilityInterestRateDescription_6F92D0F9304FC2FB429307942201F1F5" xlink:type="locator" />
    <definitionArc order="20" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_us-gaap_LineOfCreditFacilityInterestRateDescription_6F92D0F9304FC2FB429307942201F1F5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityCommitmentFeeDescription" xlink:label="loc_us-gaap_LineOfCreditFacilityCommitmentFeeDescription_A208C60B0502F192F97B079422027D2A" xlink:type="locator" />
    <definitionArc order="21" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_us-gaap_LineOfCreditFacilityCommitmentFeeDescription_A208C60B0502F192F97B079422027D2A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding" xlink:label="loc_us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding_06A136E66007BE88365407942202724D" xlink:type="locator" />
    <definitionArc order="22" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding_06A136E66007BE88365407942202724D" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Longtermdebtissuancecosts" xlink:label="loc_lh_Longtermdebtissuancecosts_7082C41E08EE2098FDEB079422026F60" xlink:type="locator" />
    <definitionArc order="23" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_lh_Longtermdebtissuancecosts_7082C41E08EE2098FDEB079422026F60" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CapitalLeaseObligationsNoncurrent" xlink:label="loc_us-gaap_CapitalLeaseObligationsNoncurrent_330D46A6A50857A4F6D807942202558B" xlink:type="locator" />
    <definitionArc order="24" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_us-gaap_CapitalLeaseObligationsNoncurrent_330D46A6A50857A4F6D807942202558B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_OtherLongTermDebt" xlink:label="loc_us-gaap_OtherLongTermDebt_C801A77C489B0D8CDA2B07942202A1A8" xlink:type="locator" />
    <definitionArc order="25" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_us-gaap_OtherLongTermDebt_C801A77C489B0D8CDA2B07942202A1A8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CapitalLeaseObligationsCurrent" xlink:label="loc_us-gaap_CapitalLeaseObligationsCurrent_84E361A742853422A4ED07942202CB17" xlink:type="locator" />
    <definitionArc order="26" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_us-gaap_CapitalLeaseObligationsCurrent_84E361A742853422A4ED07942202CB17" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtCurrent" xlink:label="loc_us-gaap_DebtCurrent_DD49411B0AC1B60FE9BF07942202E05B" xlink:type="locator" />
    <definitionArc order="27" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_8A6E5E34669CBDB9EFCA07942201927B" xlink:to="loc_us-gaap_DebtCurrent_DD49411B0AC1B60FE9BF07942202E05B" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/DebtShortTermBorrowingsAndCurrentPortionOfLongTermDebtTableDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShortTermDebtLineItems" xlink:label="loc_us-gaap_ShortTermDebtLineItems_0925791938FB51E2930C3AFF487CA33D" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfShortTermDebtTable" xlink:label="loc_us-gaap_ScheduleOfShortTermDebtTable_23D77F285FB35DE6A1D1CE088246E01B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShortTermDebtLineItems_0925791938FB51E2930C3AFF487CA33D" xlink:to="loc_us-gaap_ScheduleOfShortTermDebtTable_23D77F285FB35DE6A1D1CE088246E01B" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_B90EE7B220BD50BA935737294DC6ACBD" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShortTermDebtTable_23D77F285FB35DE6A1D1CE088246E01B" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_B90EE7B220BD50BA935737294DC6ACBD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_3AA8D9E5A78EF686B2A008E12FD274F8_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_B90EE7B220BD50BA935737294DC6ACBD" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_3AA8D9E5A78EF686B2A008E12FD274F8_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_60AA0D33685D5C6EB2286CF32516C21C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_B90EE7B220BD50BA935737294DC6ACBD" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_60AA0D33685D5C6EB2286CF32516C21C" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CovanceMember" xlink:label="loc_lh_CovanceMember_61937F6FC5C659BABDFD590CCA4A9F47" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_60AA0D33685D5C6EB2286CF32516C21C" xlink:to="loc_lh_CovanceMember_61937F6FC5C659BABDFD590CCA4A9F47" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_0D9FD5ED3D645884B6AB649533A7F25F" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShortTermDebtTable_23D77F285FB35DE6A1D1CE088246E01B" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_0D9FD5ED3D645884B6AB649533A7F25F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_650C91619BCC2C4BFA5408E12FD2B455_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_0D9FD5ED3D645884B6AB649533A7F25F" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_650C91619BCC2C4BFA5408E12FD2B455_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_305973A31DFC51A5B222966F45013C42" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_0D9FD5ED3D645884B6AB649533A7F25F" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_305973A31DFC51A5B222966F45013C42" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="loc_us-gaap_CreditFacilityAxis_0CE0B40B48F75A389677E0A39392B524" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShortTermDebtTable_23D77F285FB35DE6A1D1CE088246E01B" xlink:to="loc_us-gaap_CreditFacilityAxis_0CE0B40B48F75A389677E0A39392B524" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_93EB48F7AC9EEC9985E208E12FD2E1A8_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_0CE0B40B48F75A389677E0A39392B524" xlink:to="loc_us-gaap_CreditFacilityDomain_93EB48F7AC9EEC9985E208E12FD2E1A8_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_3C980CA281695F2087B7A8904DA9FD83" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_0CE0B40B48F75A389677E0A39392B524" xlink:to="loc_us-gaap_CreditFacilityDomain_3C980CA281695F2087B7A8904DA9FD83" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_A60DayDebtBridgeTrauncheDomain" xlink:label="loc_lh_A60DayDebtBridgeTrauncheDomain_B0C1D961EEA75AACA8C10CC4FC89E545" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_3C980CA281695F2087B7A8904DA9FD83" xlink:to="loc_lh_A60DayDebtBridgeTrauncheDomain_B0C1D961EEA75AACA8C10CC4FC89E545" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_9DE67FA759B058B6B1A92D2F52EA0B12" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShortTermDebtTable_23D77F285FB35DE6A1D1CE088246E01B" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_9DE67FA759B058B6B1A92D2F52EA0B12" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_57EFDB2BC3A25B7E7D9F08E12FD24C30_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_9DE67FA759B058B6B1A92D2F52EA0B12" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_57EFDB2BC3A25B7E7D9F08E12FD24C30_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_0621E08B18E95D32A26FAE667D08E7BE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_9DE67FA759B058B6B1A92D2F52EA0B12" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_0621E08B18E95D32A26FAE667D08E7BE" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_SeniorNotesDue2020Member" xlink:label="loc_lh_SeniorNotesDue2020Member_F439819EC29A5B4188E09CCF963855B2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_0621E08B18E95D32A26FAE667D08E7BE" xlink:to="loc_lh_SeniorNotesDue2020Member_F439819EC29A5B4188E09CCF963855B2" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_SeniorNotesDue2022Member" xlink:label="loc_lh_SeniorNotesDue2022Member_C55859D13BAA5101B0290CCEDD499844" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_0621E08B18E95D32A26FAE667D08E7BE" xlink:to="loc_lh_SeniorNotesDue2022Member_C55859D13BAA5101B0290CCEDD499844" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Seniornotesdue2025Member" xlink:label="loc_lh_Seniornotesdue2025Member_EA86F8464ED452D682B48C565365E185" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_0621E08B18E95D32A26FAE667D08E7BE" xlink:to="loc_lh_Seniornotesdue2025Member_EA86F8464ED452D682B48C565365E185" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Seniornotesdue2045Member" xlink:label="loc_lh_Seniornotesdue2045Member_8D22C5D413215D53938FB68914D45664" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_0621E08B18E95D32A26FAE667D08E7BE" xlink:to="loc_lh_Seniornotesdue2045Member_8D22C5D413215D53938FB68914D45664" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShortTermDebtTypeAxis" xlink:label="loc_us-gaap_ShortTermDebtTypeAxis_27E241CB8DFC5B7CB217CAD21C2CA2AA" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShortTermDebtTable_23D77F285FB35DE6A1D1CE088246E01B" xlink:to="loc_us-gaap_ShortTermDebtTypeAxis_27E241CB8DFC5B7CB217CAD21C2CA2AA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShortTermDebtTypeDomain" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_C006B511F8D642ADDEAB08E12FD22F6B_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_27E241CB8DFC5B7CB217CAD21C2CA2AA" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_C006B511F8D642ADDEAB08E12FD22F6B_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShortTermDebtTypeDomain" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_9DC19ECF0BA0502D9043150120934C62" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_27E241CB8DFC5B7CB217CAD21C2CA2AA" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_9DC19ECF0BA0502D9043150120934C62" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConvertibleSubordinatedDebtMember" xlink:label="loc_us-gaap_ConvertibleSubordinatedDebtMember_DB208E9ADA3D59D5BB317AFE82AC46BA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_9DC19ECF0BA0502D9043150120934C62" xlink:to="loc_us-gaap_ConvertibleSubordinatedDebtMember_DB208E9ADA3D59D5BB317AFE82AC46BA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SeniorLongTermNotes" xlink:label="loc_us-gaap_SeniorLongTermNotes_BFBC5D5EA38E5B65BD13D3694DEC7712" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_0925791938FB51E2930C3AFF487CA33D" xlink:to="loc_us-gaap_SeniorLongTermNotes_BFBC5D5EA38E5B65BD13D3694DEC7712" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019ECDFC069850B4A84D3DD5A4AB0929" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_0925791938FB51E2930C3AFF487CA33D" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019ECDFC069850B4A84D3DD5A4AB0929" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_BridgeTermCreditFacilityAgreementMaximumBorrowingAmount" xlink:label="loc_lh_BridgeTermCreditFacilityAgreementMaximumBorrowingAmount_E25F140FC4C556BABC7B3FEAE6B88D3B" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_0925791938FB51E2930C3AFF487CA33D" xlink:to="loc_lh_BridgeTermCreditFacilityAgreementMaximumBorrowingAmount_E25F140FC4C556BABC7B3FEAE6B88D3B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CapitalLeaseObligationsCurrent" xlink:label="loc_us-gaap_CapitalLeaseObligationsCurrent_1A909087C059579C9E9B2CD4A7DFE194" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_0925791938FB51E2930C3AFF487CA33D" xlink:to="loc_us-gaap_CapitalLeaseObligationsCurrent_1A909087C059579C9E9B2CD4A7DFE194" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtCurrent" xlink:label="loc_us-gaap_DebtCurrent_D61F5F4637C8526C8024FFAEF37E2F62" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_0925791938FB51E2930C3AFF487CA33D" xlink:to="loc_us-gaap_DebtCurrent_D61F5F4637C8526C8024FFAEF37E2F62" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/DebtTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/DerivativeInstrumentsAndHedgingActivities" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/DerivativeInstrumentsAndHedgingActivitiesDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DerivativeInstrumentsGainLossLineItems" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_D18CE804B18C520087D6F9F30E312B20" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_0DFA0F9307CD50BA8525608390231C91" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_D18CE804B18C520087D6F9F30E312B20" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_0DFA0F9307CD50BA8525608390231C91" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_36D9E05CDCBA5489BE4777B5A4B2F2B4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_0DFA0F9307CD50BA8525608390231C91" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_36D9E05CDCBA5489BE4777B5A4B2F2B4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_894404E6B555819E05EFC94032297D2B_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_36D9E05CDCBA5489BE4777B5A4B2F2B4" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_894404E6B555819E05EFC94032297D2B_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_FD23616B870257C7BFCCC70B3F4A3415" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_36D9E05CDCBA5489BE4777B5A4B2F2B4" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_FD23616B870257C7BFCCC70B3F4A3415" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_SeniorNotesDue2020Member" xlink:label="loc_lh_SeniorNotesDue2020Member_4B4C3C08F54D5B75A202FC6577BA333D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_FD23616B870257C7BFCCC70B3F4A3415" xlink:to="loc_lh_SeniorNotesDue2020Member_4B4C3C08F54D5B75A202FC6577BA333D" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_SeniorLongTermNotesDue2020Member" xlink:label="loc_lh_SeniorLongTermNotesDue2020Member_3411BF3CAAA851E5AEE94B6873519B1F" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_FD23616B870257C7BFCCC70B3F4A3415" xlink:to="loc_lh_SeniorLongTermNotesDue2020Member_3411BF3CAAA851E5AEE94B6873519B1F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DerivativeInstrumentRiskAxis" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_CFB2DEFAF3BE54BD808037BACD6155CB" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_0DFA0F9307CD50BA8525608390231C91" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_CFB2DEFAF3BE54BD808037BACD6155CB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_3EF5436239195B3BE3B1C9403229136C_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_CFB2DEFAF3BE54BD808037BACD6155CB" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_3EF5436239195B3BE3B1C9403229136C_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_7A08AE2840335D28A09D34C5A5DDC9B2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_CFB2DEFAF3BE54BD808037BACD6155CB" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_7A08AE2840335D28A09D34C5A5DDC9B2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_8B48689D28EF580394E961C094077911" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_D18CE804B18C520087D6F9F30E312B20" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_8B48689D28EF580394E961C094077911" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongTermDebtNoncurrent" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_5C82BA9FC947596E9140071FB4C30E6A" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_D18CE804B18C520087D6F9F30E312B20" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_5C82BA9FC947596E9140071FB4C30E6A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_99329291AADC520FA981B9D64E12BCD6" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_D18CE804B18C520087D6F9F30E312B20" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_99329291AADC520FA981B9D64E12BCD6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueHedgesAtFairValueNet" xlink:label="loc_us-gaap_FairValueHedgesAtFairValueNet_08707C7477D65870A2CE1A35DA600F62" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_D18CE804B18C520087D6F9F30E312B20" xlink:to="loc_us-gaap_FairValueHedgesAtFairValueNet_08707C7477D65870A2CE1A35DA600F62" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_MinimumPercentageMarketToCalculatedPriceAtWhichContingentCashInterestIsDue" xlink:label="loc_lh_MinimumPercentageMarketToCalculatedPriceAtWhichContingentCashInterestIsDue_EC51FE4729BB5BC1A4386B00F66F3FB8" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_D18CE804B18C520087D6F9F30E312B20" xlink:to="loc_lh_MinimumPercentageMarketToCalculatedPriceAtWhichContingentCashInterestIsDue_EC51FE4729BB5BC1A4386B00F66F3FB8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_EmbeddedDerivativeFairValueOfEmbeddedDerivativeNet" xlink:label="loc_us-gaap_EmbeddedDerivativeFairValueOfEmbeddedDerivativeNet_0D76A2309FDE52E4B67AEA3EE6C1A651" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_D18CE804B18C520087D6F9F30E312B20" xlink:to="loc_us-gaap_EmbeddedDerivativeFairValueOfEmbeddedDerivativeNet_0D76A2309FDE52E4B67AEA3EE6C1A651" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_EmbeddedDerivativeGainLossOnEmbeddedDerivativeNet" xlink:label="loc_us-gaap_EmbeddedDerivativeGainLossOnEmbeddedDerivativeNet_57E7367369F4515BBF8D32D2F970FC35" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_D18CE804B18C520087D6F9F30E312B20" xlink:to="loc_us-gaap_EmbeddedDerivativeGainLossOnEmbeddedDerivativeNet_57E7367369F4515BBF8D32D2F970FC35" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/DerivativeInstrumentsAndHedgingActivitiesTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/DocumentAndEntityInformationDocument" xlink:type="extended">
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentInformationLineItems" xlink:label="loc_dei_DocumentInformationLineItems_265502C99A8E5BEBABD0086B77302E6A" xlink:type="locator" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentInformationTable" xlink:label="loc_dei_DocumentInformationTable_0772BD62DAA25FF98FA620301AC042CB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_265502C99A8E5BEBABD0086B77302E6A" xlink:to="loc_dei_DocumentInformationTable_0772BD62DAA25FF98FA620301AC042CB" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentInformationDocumentAxis" xlink:label="loc_dei_DocumentInformationDocumentAxis_4040BB63EE7855D48AF802B5F6718CB5" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_0772BD62DAA25FF98FA620301AC042CB" xlink:to="loc_dei_DocumentInformationDocumentAxis_4040BB63EE7855D48AF802B5F6718CB5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentDomain" xlink:label="loc_dei_DocumentDomain_1B35311C36B672A60843C940316BE953_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_DocumentInformationDocumentAxis_4040BB63EE7855D48AF802B5F6718CB5" xlink:to="loc_dei_DocumentDomain_1B35311C36B672A60843C940316BE953_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentDomain" xlink:label="loc_dei_DocumentDomain_B23E8B451B5B56768BC63FF0D7F6F092" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_DocumentInformationDocumentAxis_4040BB63EE7855D48AF802B5F6718CB5" xlink:to="loc_dei_DocumentDomain_B23E8B451B5B56768BC63FF0D7F6F092" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityRegistrantName" xlink:label="loc_dei_EntityRegistrantName_CBE13873F9415BA7BBDBA62871E8103B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_265502C99A8E5BEBABD0086B77302E6A" xlink:to="loc_dei_EntityRegistrantName_CBE13873F9415BA7BBDBA62871E8103B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityCentralIndexKey" xlink:label="loc_dei_EntityCentralIndexKey_E5B1FDE1D64357649A01FA73BFC5E664" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_265502C99A8E5BEBABD0086B77302E6A" xlink:to="loc_dei_EntityCentralIndexKey_E5B1FDE1D64357649A01FA73BFC5E664" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_CurrentFiscalYearEndDate" xlink:label="loc_dei_CurrentFiscalYearEndDate_5BC46C1C0A1059A58DF32D80EAD429B1" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_265502C99A8E5BEBABD0086B77302E6A" xlink:to="loc_dei_CurrentFiscalYearEndDate_5BC46C1C0A1059A58DF32D80EAD429B1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityFilerCategory" xlink:label="loc_dei_EntityFilerCategory_EE79268DE898501DA6F980F31DC744DF" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_265502C99A8E5BEBABD0086B77302E6A" xlink:to="loc_dei_EntityFilerCategory_EE79268DE898501DA6F980F31DC744DF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentType" xlink:label="loc_dei_DocumentType_36B1AE2356BC5245AAFA8112E3CAD116" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_265502C99A8E5BEBABD0086B77302E6A" xlink:to="loc_dei_DocumentType_36B1AE2356BC5245AAFA8112E3CAD116" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentPeriodEndDate" xlink:label="loc_dei_DocumentPeriodEndDate_CB2D3552FDB851AAB61060E4BD7E7BAD" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_265502C99A8E5BEBABD0086B77302E6A" xlink:to="loc_dei_DocumentPeriodEndDate_CB2D3552FDB851AAB61060E4BD7E7BAD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentFiscalYearFocus" xlink:label="loc_dei_DocumentFiscalYearFocus_78F1D040557D5B2DA549B6E670D62E37" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_265502C99A8E5BEBABD0086B77302E6A" xlink:to="loc_dei_DocumentFiscalYearFocus_78F1D040557D5B2DA549B6E670D62E37" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentFiscalPeriodFocus" xlink:label="loc_dei_DocumentFiscalPeriodFocus_90A5F5C62E165F53A05010CEA99F0818" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_265502C99A8E5BEBABD0086B77302E6A" xlink:to="loc_dei_DocumentFiscalPeriodFocus_90A5F5C62E165F53A05010CEA99F0818" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_AmendmentFlag" xlink:label="loc_dei_AmendmentFlag_DCCBE436FC1355A797F631083417650F" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_265502C99A8E5BEBABD0086B77302E6A" xlink:to="loc_dei_AmendmentFlag_DCCBE436FC1355A797F631083417650F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityCommonStockSharesOutstanding" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_27B80ACDFACA5978867149358A2E917F" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_265502C99A8E5BEBABD0086B77302E6A" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_27B80ACDFACA5978867149358A2E917F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityWellKnownSeasonedIssuer" xlink:label="loc_dei_EntityWellKnownSeasonedIssuer_C4202E864DA0586DB29D8D3E4A3D746F" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_265502C99A8E5BEBABD0086B77302E6A" xlink:to="loc_dei_EntityWellKnownSeasonedIssuer_C4202E864DA0586DB29D8D3E4A3D746F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityVoluntaryFilers" xlink:label="loc_dei_EntityVoluntaryFilers_B3C9B328F72D596EAD6AB6189CC0849D" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_265502C99A8E5BEBABD0086B77302E6A" xlink:to="loc_dei_EntityVoluntaryFilers_B3C9B328F72D596EAD6AB6189CC0849D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityCurrentReportingStatus" xlink:label="loc_dei_EntityCurrentReportingStatus_FF9D1238E6FE5411A30C2202F8FB7091" xlink:type="locator" />
    <definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_265502C99A8E5BEBABD0086B77302E6A" xlink:to="loc_dei_EntityCurrentReportingStatus_FF9D1238E6FE5411A30C2202F8FB7091" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/EarningsPerShare" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/EarningsPerSharePotentialCommonSharesNotIncludedInComputationOfDilutedEarningsPerShareDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/EarningsPerShareReconciliationOfBasicEarningsPerShareToDilutedEarningsPerShareDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/EarningsPerShareTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/FairValueMeasurements" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/FairValueMeasurementsDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_6CA5BC5D028A57F4A6E3A90AF6003E04" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_6D0A2162B5E851D9BB12AE0132A1BE88" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_6CA5BC5D028A57F4A6E3A90AF6003E04" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_6D0A2162B5E851D9BB12AE0132A1BE88" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_243FAA53FD475F63BF180B3EDE971B04" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_6D0A2162B5E851D9BB12AE0132A1BE88" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_243FAA53FD475F63BF180B3EDE971B04" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_9AC13E39DF2D20D41FEEC9403341FED6_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_243FAA53FD475F63BF180B3EDE971B04" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_9AC13E39DF2D20D41FEEC9403341FED6_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_4FF9789B781C5395A25118F586FACEC7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_243FAA53FD475F63BF180B3EDE971B04" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_4FF9789B781C5395A25118F586FACEC7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_68C6DF076349581BBFCC3B9E6BB8B453" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_4FF9789B781C5395A25118F586FACEC7" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_68C6DF076349581BBFCC3B9E6BB8B453" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_33B5314C857E58DF8AA81C497B83442B" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_4FF9789B781C5395A25118F586FACEC7" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_33B5314C857E58DF8AA81C497B83442B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_C83BA1450E715D9783B2914CCB0099C2" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_4FF9789B781C5395A25118F586FACEC7" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_C83BA1450E715D9783B2914CCB0099C2" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_FairValueLiabilitiesMeasuredOnRecurringBasisNoncontrollingInterestPuts" xlink:label="loc_lh_FairValueLiabilitiesMeasuredOnRecurringBasisNoncontrollingInterestPuts_14365C191EC15CB2A52D11E2058C132F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_6CA5BC5D028A57F4A6E3A90AF6003E04" xlink:to="loc_lh_FairValueLiabilitiesMeasuredOnRecurringBasisNoncontrollingInterestPuts_14365C191EC15CB2A52D11E2058C132F" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_ZeroCouponNotesFairValue" xlink:label="loc_lh_ZeroCouponNotesFairValue_C48B33138B1C597291505F8426B53AA8" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_6CA5BC5D028A57F4A6E3A90AF6003E04" xlink:to="loc_lh_ZeroCouponNotesFairValue_C48B33138B1C597291505F8426B53AA8" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_SeniorNotesFairValue" xlink:label="loc_lh_SeniorNotesFairValue_1263BCDD379B59248A1B8A3CA3E71142" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_6CA5BC5D028A57F4A6E3A90AF6003E04" xlink:to="loc_lh_SeniorNotesFairValue_1263BCDD379B59248A1B8A3CA3E71142" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CashSurrenderValueFairValueDisclosure" xlink:label="loc_us-gaap_CashSurrenderValueFairValueDisclosure_C26376DE2A0C560B88132C3E90075F1B" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_6CA5BC5D028A57F4A6E3A90AF6003E04" xlink:to="loc_us-gaap_CashSurrenderValueFairValueDisclosure_C26376DE2A0C560B88132C3E90075F1B" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_FairValueLiabilitiesMeasuredOnRecurringBasisDeferredCompensationLiability" xlink:label="loc_lh_FairValueLiabilitiesMeasuredOnRecurringBasisDeferredCompensationLiability_770F9BF58F0E520EA06B2C8C3189C8B6" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_6CA5BC5D028A57F4A6E3A90AF6003E04" xlink:to="loc_lh_FairValueLiabilitiesMeasuredOnRecurringBasisDeferredCompensationLiability_770F9BF58F0E520EA06B2C8C3189C8B6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueHedgesAtFairValueNet" xlink:label="loc_us-gaap_FairValueHedgesAtFairValueNet_B7943FDCB36F5B66A50B7257CD338BF1" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_6CA5BC5D028A57F4A6E3A90AF6003E04" xlink:to="loc_us-gaap_FairValueHedgesAtFairValueNet_B7943FDCB36F5B66A50B7257CD338BF1" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/FairValueMeasurementsTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/GoodwillAndIntangibleAssets" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/GoodwillAndIntangibleAssetsChangesInCarryingAmountOfGoodwillDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_GoodwillLineItems" xlink:label="loc_us-gaap_GoodwillLineItems_536BA1EC39895CC4A48029C3C6EC9551" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfGoodwillTable" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_611EF2CD071C556CB60826B9F7D115A6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_536BA1EC39895CC4A48029C3C6EC9551" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_611EF2CD071C556CB60826B9F7D115A6" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="lh-20160930.xsd#lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis" xlink:label="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis_57FA31AFCE7653A49EEFEB15505A1003" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_611EF2CD071C556CB60826B9F7D115A6" xlink:to="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis_57FA31AFCE7653A49EEFEB15505A1003" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain" xlink:label="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_E120D77F3BD6F8EA5278C94032472E08_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis_57FA31AFCE7653A49EEFEB15505A1003" xlink:to="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_E120D77F3BD6F8EA5278C94032472E08_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain" xlink:label="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_65393A2F864C5812BA039477690BEFC9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis_57FA31AFCE7653A49EEFEB15505A1003" xlink:to="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_65393A2F864C5812BA039477690BEFC9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CustomerRelationshipsMember" xlink:label="loc_us-gaap_CustomerRelationshipsMember_6E91FED7676D54B296AA310B6324CDD2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_65393A2F864C5812BA039477690BEFC9" xlink:to="loc_us-gaap_CustomerRelationshipsMember_6E91FED7676D54B296AA310B6324CDD2" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_PatentsLicensesAndTechnologyMember" xlink:label="loc_lh_PatentsLicensesAndTechnologyMember_83C9E47F67525F56B5881E5FBDB64FA0" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_65393A2F864C5812BA039477690BEFC9" xlink:to="loc_lh_PatentsLicensesAndTechnologyMember_83C9E47F67525F56B5881E5FBDB64FA0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_NoncompeteAgreementsMember" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_066429CF43435787AE714F2224C54618" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_65393A2F864C5812BA039477690BEFC9" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_066429CF43435787AE714F2224C54618" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_TradeNamesMember" xlink:label="loc_us-gaap_TradeNamesMember_380FB0F13C7E52D1B4FD763B4CFCB9C6" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_65393A2F864C5812BA039477690BEFC9" xlink:to="loc_us-gaap_TradeNamesMember_380FB0F13C7E52D1B4FD763B4CFCB9C6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_UseRightsMember" xlink:label="loc_us-gaap_UseRightsMember_CE0B619E1E7358FA94EB9F1C1E3E5F27" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_65393A2F864C5812BA039477690BEFC9" xlink:to="loc_us-gaap_UseRightsMember_CE0B619E1E7358FA94EB9F1C1E3E5F27" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LicensingAgreementsMember" xlink:label="loc_us-gaap_LicensingAgreementsMember_736C405534FD59AB92A12ADF3F8F5BE2" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_65393A2F864C5812BA039477690BEFC9" xlink:to="loc_us-gaap_LicensingAgreementsMember_736C405534FD59AB92A12ADF3F8F5BE2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConsolidationItemsAxis" xlink:label="loc_us-gaap_ConsolidationItemsAxis_FA4775173F4C52DAACBB3D9086B0C3CA" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_611EF2CD071C556CB60826B9F7D115A6" xlink:to="loc_us-gaap_ConsolidationItemsAxis_FA4775173F4C52DAACBB3D9086B0C3CA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConsolidationItemsDomain" xlink:label="loc_us-gaap_ConsolidationItemsDomain_CC8C4F5DA7217C6D5BEAC9403247AF73_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConsolidationItemsAxis_FA4775173F4C52DAACBB3D9086B0C3CA" xlink:to="loc_us-gaap_ConsolidationItemsDomain_CC8C4F5DA7217C6D5BEAC9403247AF73_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConsolidationItemsDomain" xlink:label="loc_us-gaap_ConsolidationItemsDomain_FEC87F27F9CF586DA772881B4B1948A7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConsolidationItemsAxis_FA4775173F4C52DAACBB3D9086B0C3CA" xlink:to="loc_us-gaap_ConsolidationItemsDomain_FEC87F27F9CF586DA772881B4B1948A7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_F89C8BD5A25E5ADA9B3548FD47A770EE" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_611EF2CD071C556CB60826B9F7D115A6" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_F89C8BD5A25E5ADA9B3548FD47A770EE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_25BD13FAA0D6BF0364AEC9403247C4FC_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_F89C8BD5A25E5ADA9B3548FD47A770EE" xlink:to="loc_us-gaap_SegmentDomain_25BD13FAA0D6BF0364AEC9403247C4FC_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_1A9477F875D55D409F1A893E4E86C267" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_F89C8BD5A25E5ADA9B3548FD47A770EE" xlink:to="loc_us-gaap_SegmentDomain_1A9477F875D55D409F1A893E4E86C267" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_LabCorpDiagnosticsMember" xlink:label="loc_lh_LabCorpDiagnosticsMember_34BA7905C6B152F8978D3A521A876E23" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_1A9477F875D55D409F1A893E4E86C267" xlink:to="loc_lh_LabCorpDiagnosticsMember_34BA7905C6B152F8978D3A521A876E23" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CovanceDrugDevelopmentMember" xlink:label="loc_lh_CovanceDrugDevelopmentMember_41736A8AB1B35F049C8DB1B03E00ECDB" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_1A9477F875D55D409F1A893E4E86C267" xlink:to="loc_lh_CovanceDrugDevelopmentMember_41736A8AB1B35F049C8DB1B03E00ECDB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_60377F34F72C55DB9D3FDD20106E1344" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_611EF2CD071C556CB60826B9F7D115A6" xlink:to="loc_us-gaap_RangeAxis_60377F34F72C55DB9D3FDD20106E1344" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_535483D43D2B041A4332C940325161B8_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_60377F34F72C55DB9D3FDD20106E1344" xlink:to="loc_us-gaap_RangeMember_535483D43D2B041A4332C940325161B8_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_91F94D0D0D9B5D5DB80F714AECFEC9AC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_60377F34F72C55DB9D3FDD20106E1344" xlink:to="loc_us-gaap_RangeMember_91F94D0D0D9B5D5DB80F714AECFEC9AC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SeveranceCosts1" xlink:label="loc_us-gaap_SeveranceCosts1_DD074047D1FA589B84970954E704D390" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_536BA1EC39895CC4A48029C3C6EC9551" xlink:to="loc_us-gaap_SeveranceCosts1_DD074047D1FA589B84970954E704D390" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IntangibleAssetsGrossExcludingGoodwill" xlink:label="loc_us-gaap_IntangibleAssetsGrossExcludingGoodwill_C86F36181FB35698AA72589B6FC5F43E" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_536BA1EC39895CC4A48029C3C6EC9551" xlink:to="loc_us-gaap_IntangibleAssetsGrossExcludingGoodwill_C86F36181FB35698AA72589B6FC5F43E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_Goodwill" xlink:label="loc_us-gaap_Goodwill_954DF6092D91524E99A571E69CEFB934" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_536BA1EC39895CC4A48029C3C6EC9551" xlink:to="loc_us-gaap_Goodwill_954DF6092D91524E99A571E69CEFB934" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_GoodwillAcquiredDuringPeriod" xlink:label="loc_us-gaap_GoodwillAcquiredDuringPeriod_9A6196C3143E5CE285D2DA8869F9A93B" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_536BA1EC39895CC4A48029C3C6EC9551" xlink:to="loc_us-gaap_GoodwillAcquiredDuringPeriod_9A6196C3143E5CE285D2DA8869F9A93B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_GoodwillTranslationAndPurchaseAccountingAdjustments" xlink:label="loc_us-gaap_GoodwillTranslationAndPurchaseAccountingAdjustments_C9A3B8DBCC5B56BD8C07CDDD8F95C95A" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_536BA1EC39895CC4A48029C3C6EC9551" xlink:to="loc_us-gaap_GoodwillTranslationAndPurchaseAccountingAdjustments_C9A3B8DBCC5B56BD8C07CDDD8F95C95A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_DE3E9B8D03735056A4C603BA430DCE4A" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_536BA1EC39895CC4A48029C3C6EC9551" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_DE3E9B8D03735056A4C603BA430DCE4A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_3BD0A74E4BC85D0C825F2E5250441C67" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_536BA1EC39895CC4A48029C3C6EC9551" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_3BD0A74E4BC85D0C825F2E5250441C67" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/GoodwillAndIntangibleAssetsComponentsOfIdentifiableIntangibleAssetsDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_61225BB1E7A250478BF0B97B23E066A9" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_D76D05FB7A055D269652B8E50CD746B1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_61225BB1E7A250478BF0B97B23E066A9" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_D76D05FB7A055D269652B8E50CD746B1" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_C68E5E3372E15C9B98DEEC073662C137" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_D76D05FB7A055D269652B8E50CD746B1" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_C68E5E3372E15C9B98DEEC073662C137" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_AEDC5291CD28A79B0A88C94030DFE911_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_C68E5E3372E15C9B98DEEC073662C137" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_AEDC5291CD28A79B0A88C94030DFE911_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_3EC67CB3F02F5E1DA2AEE0AE246A9C66" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_C68E5E3372E15C9B98DEEC073662C137" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_3EC67CB3F02F5E1DA2AEE0AE246A9C66" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis" xlink:label="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis_B27371F46D545191886640C5C912D149" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_D76D05FB7A055D269652B8E50CD746B1" xlink:to="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis_B27371F46D545191886640C5C912D149" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain" xlink:label="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_70F9716D85934727E2BFC94030DF3BE1_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis_B27371F46D545191886640C5C912D149" xlink:to="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_70F9716D85934727E2BFC94030DF3BE1_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain" xlink:label="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_19F68F43B4E950EA9DBA31EC4934D07F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis_B27371F46D545191886640C5C912D149" xlink:to="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_19F68F43B4E950EA9DBA31EC4934D07F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CustomerRelationshipsMember" xlink:label="loc_us-gaap_CustomerRelationshipsMember_4CDEDD5F82465E40B12B6A71A1085F3A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_19F68F43B4E950EA9DBA31EC4934D07F" xlink:to="loc_us-gaap_CustomerRelationshipsMember_4CDEDD5F82465E40B12B6A71A1085F3A" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_PatentsLicensesAndTechnologyMember" xlink:label="loc_lh_PatentsLicensesAndTechnologyMember_BE92D6118939586698E08D99C87BEC24" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_19F68F43B4E950EA9DBA31EC4934D07F" xlink:to="loc_lh_PatentsLicensesAndTechnologyMember_BE92D6118939586698E08D99C87BEC24" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_NoncompeteAgreementsMember" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_9EB6DEAD76E657168F4DEE7F6EBBABA3" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_19F68F43B4E950EA9DBA31EC4934D07F" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_9EB6DEAD76E657168F4DEE7F6EBBABA3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_TradeNamesMember" xlink:label="loc_us-gaap_TradeNamesMember_73C7479028E759A78F518211384F33B9" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_19F68F43B4E950EA9DBA31EC4934D07F" xlink:to="loc_us-gaap_TradeNamesMember_73C7479028E759A78F518211384F33B9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_UseRightsMember" xlink:label="loc_us-gaap_UseRightsMember_5203164B0BE053458232CF007422E369" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_19F68F43B4E950EA9DBA31EC4934D07F" xlink:to="loc_us-gaap_UseRightsMember_5203164B0BE053458232CF007422E369" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LicensingAgreementsMember" xlink:label="loc_us-gaap_LicensingAgreementsMember_5C954264BAE3566D8F76753A8DC9A323" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_19F68F43B4E950EA9DBA31EC4934D07F" xlink:to="loc_us-gaap_LicensingAgreementsMember_5C954264BAE3566D8F76753A8DC9A323" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IntangibleAssetsGrossExcludingGoodwill" xlink:label="loc_us-gaap_IntangibleAssetsGrossExcludingGoodwill_361CB1BD4E6F5B618ACE38FD3C4E5D4E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_61225BB1E7A250478BF0B97B23E066A9" xlink:to="loc_us-gaap_IntangibleAssetsGrossExcludingGoodwill_361CB1BD4E6F5B618ACE38FD3C4E5D4E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_95CED5741A9D514D88CA708204528617" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_61225BB1E7A250478BF0B97B23E066A9" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_95CED5741A9D514D88CA708204528617" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_8695D2686A51568EADF599CB7F28627F" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_61225BB1E7A250478BF0B97B23E066A9" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_8695D2686A51568EADF599CB7F28627F" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/GoodwillAndIntangibleAssetsDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_EAE8BE858751576F91755289F08F4E79" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_878F587E981151CE94D2A9216D44EBFD" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_EAE8BE858751576F91755289F08F4E79" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_878F587E981151CE94D2A9216D44EBFD" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_C6054F8ADE7458BAB8C38E49B28C6137" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_878F587E981151CE94D2A9216D44EBFD" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_C6054F8ADE7458BAB8C38E49B28C6137" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_379D194787D0A534E046E3276CD41075_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_C6054F8ADE7458BAB8C38E49B28C6137" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_379D194787D0A534E046E3276CD41075_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_E4B09DC7294B5DDD8F55F3D69A1560A2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_C6054F8ADE7458BAB8C38E49B28C6137" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_E4B09DC7294B5DDD8F55F3D69A1560A2" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Seniornotesdue2045Member" xlink:label="loc_lh_Seniornotesdue2045Member_7359BBF0E6435CA89469ECBDEED9700F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_E4B09DC7294B5DDD8F55F3D69A1560A2" xlink:to="loc_lh_Seniornotesdue2045Member_7359BBF0E6435CA89469ECBDEED9700F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_0F27A3736D7453E4ACAB9CB36BAD72A7" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_878F587E981151CE94D2A9216D44EBFD" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_0F27A3736D7453E4ACAB9CB36BAD72A7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F49469E5AB427D737FFEE3276CD4899C_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_0F27A3736D7453E4ACAB9CB36BAD72A7" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F49469E5AB427D737FFEE3276CD4899C_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_6C9279DBF0BD514CAD104A66050AA050" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_0F27A3736D7453E4ACAB9CB36BAD72A7" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_6C9279DBF0BD514CAD104A66050AA050" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CovanceMember" xlink:label="loc_lh_CovanceMember_40880E1F305B5F8FBBBB7C8BCED495CA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_6C9279DBF0BD514CAD104A66050AA050" xlink:to="loc_lh_CovanceMember_40880E1F305B5F8FBBBB7C8BCED495CA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_AB7DAFD36EC4588F803756AED454A4E2" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_878F587E981151CE94D2A9216D44EBFD" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_AB7DAFD36EC4588F803756AED454A4E2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_E5DD71E0B9D675325F17E3276CD432A8_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_AB7DAFD36EC4588F803756AED454A4E2" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_E5DD71E0B9D675325F17E3276CD432A8_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_43D0D1BC8C1A5819BD26947BEB78EE33" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_AB7DAFD36EC4588F803756AED454A4E2" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_43D0D1BC8C1A5819BD26947BEB78EE33" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConsolidationItemsAxis" xlink:label="loc_us-gaap_ConsolidationItemsAxis_D88B811052DF5118B08D8CE0DB5B177B" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_878F587E981151CE94D2A9216D44EBFD" xlink:to="loc_us-gaap_ConsolidationItemsAxis_D88B811052DF5118B08D8CE0DB5B177B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConsolidationItemsDomain" xlink:label="loc_us-gaap_ConsolidationItemsDomain_4253FF29F8A038762378E3276CD4B6A2_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConsolidationItemsAxis_D88B811052DF5118B08D8CE0DB5B177B" xlink:to="loc_us-gaap_ConsolidationItemsDomain_4253FF29F8A038762378E3276CD4B6A2_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConsolidationItemsDomain" xlink:label="loc_us-gaap_ConsolidationItemsDomain_39FB9A82A6C85A54BE2040322A20EC85" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConsolidationItemsAxis_D88B811052DF5118B08D8CE0DB5B177B" xlink:to="loc_us-gaap_ConsolidationItemsDomain_39FB9A82A6C85A54BE2040322A20EC85" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_EA0758B4B9535D1A81D6791B6E075F55" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_878F587E981151CE94D2A9216D44EBFD" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_EA0758B4B9535D1A81D6791B6E075F55" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_C002D6D8718F8AC59982E3276CD4C291_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_EA0758B4B9535D1A81D6791B6E075F55" xlink:to="loc_us-gaap_SegmentDomain_C002D6D8718F8AC59982E3276CD4C291_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_F015F3A0AEB7589A8D681E5834D35594" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_EA0758B4B9535D1A81D6791B6E075F55" xlink:to="loc_us-gaap_SegmentDomain_F015F3A0AEB7589A8D681E5834D35594" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_LabCorpDiagnosticsMember" xlink:label="loc_lh_LabCorpDiagnosticsMember_97703D2307525714B6C24A493FB99842" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_F015F3A0AEB7589A8D681E5834D35594" xlink:to="loc_lh_LabCorpDiagnosticsMember_97703D2307525714B6C24A493FB99842" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CovanceDrugDevelopmentMember" xlink:label="loc_lh_CovanceDrugDevelopmentMember_67BFCCF1C83B503ABCE56C6F7E369B8D" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_F015F3A0AEB7589A8D681E5834D35594" xlink:to="loc_lh_CovanceDrugDevelopmentMember_67BFCCF1C83B503ABCE56C6F7E369B8D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_87139354963C55D8B602AE6B38693AE7" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_878F587E981151CE94D2A9216D44EBFD" xlink:to="loc_us-gaap_RangeAxis_87139354963C55D8B602AE6B38693AE7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_654A2BB496ADB03710EFE3276CD4D85F_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_87139354963C55D8B602AE6B38693AE7" xlink:to="loc_us-gaap_RangeMember_654A2BB496ADB03710EFE3276CD4D85F_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_4921FECB23E959CD832FDC4EA02E66F1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_87139354963C55D8B602AE6B38693AE7" xlink:to="loc_us-gaap_RangeMember_4921FECB23E959CD832FDC4EA02E66F1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SeniorLongTermNotes" xlink:label="loc_us-gaap_SeniorLongTermNotes_4F947CB501665EFA89657B4AE9D3976F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_EAE8BE858751576F91755289F08F4E79" xlink:to="loc_us-gaap_SeniorLongTermNotes_4F947CB501665EFA89657B4AE9D3976F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_Goodwill" xlink:label="loc_us-gaap_Goodwill_5BC87C319B3E5308B41FABB8A03949E8" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_EAE8BE858751576F91755289F08F4E79" xlink:to="loc_us-gaap_Goodwill_5BC87C319B3E5308B41FABB8A03949E8" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/GoodwillAndIntangibleAssetsTables" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_73D3337CE2E2585D88960CFEC922834F" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_9A13A9BD1E0F5B2196583DDFC02F5B2D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_73D3337CE2E2585D88960CFEC922834F" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_9A13A9BD1E0F5B2196583DDFC02F5B2D" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_59056D6BFF3A5671A083D5B2B9813BEA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_9A13A9BD1E0F5B2196583DDFC02F5B2D" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_59056D6BFF3A5671A083D5B2B9813BEA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_9D2DF25FF01104C42AE408AAECBFF023_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_59056D6BFF3A5671A083D5B2B9813BEA" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_9D2DF25FF01104C42AE408AAECBFF023_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_85DAE5AAABCD5816989BD8D58EC6760D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_59056D6BFF3A5671A083D5B2B9813BEA" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_85DAE5AAABCD5816989BD8D58EC6760D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConsolidationItemsAxis" xlink:label="loc_us-gaap_ConsolidationItemsAxis_A0132F8BCE1353DBA7F02A9154F125C5" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_9A13A9BD1E0F5B2196583DDFC02F5B2D" xlink:to="loc_us-gaap_ConsolidationItemsAxis_A0132F8BCE1353DBA7F02A9154F125C5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConsolidationItemsDomain" xlink:label="loc_us-gaap_ConsolidationItemsDomain_AD8713E4244AB3DEBD8408AAECBF8B9B_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConsolidationItemsAxis_A0132F8BCE1353DBA7F02A9154F125C5" xlink:to="loc_us-gaap_ConsolidationItemsDomain_AD8713E4244AB3DEBD8408AAECBF8B9B_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConsolidationItemsDomain" xlink:label="loc_us-gaap_ConsolidationItemsDomain_19793CFFE8E05FD0A096B2BD0DC61D20" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConsolidationItemsAxis_A0132F8BCE1353DBA7F02A9154F125C5" xlink:to="loc_us-gaap_ConsolidationItemsDomain_19793CFFE8E05FD0A096B2BD0DC61D20" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_2A7C2729D6A0577BBDFA2A304DC07435" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_9A13A9BD1E0F5B2196583DDFC02F5B2D" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_2A7C2729D6A0577BBDFA2A304DC07435" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_3A11FB487C6A7110E7D708AAECBFC7C1_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_2A7C2729D6A0577BBDFA2A304DC07435" xlink:to="loc_us-gaap_SegmentDomain_3A11FB487C6A7110E7D708AAECBFC7C1_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_AD60080C1977549985C33A5CDC055A1E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_2A7C2729D6A0577BBDFA2A304DC07435" xlink:to="loc_us-gaap_SegmentDomain_AD60080C1977549985C33A5CDC055A1E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_4A94A5360DE3576E8415C248130A0DE8" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_9A13A9BD1E0F5B2196583DDFC02F5B2D" xlink:to="loc_us-gaap_RangeAxis_4A94A5360DE3576E8415C248130A0DE8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_5FFA828BE2B903BAD04708AAECC00A50_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_4A94A5360DE3576E8415C248130A0DE8" xlink:to="loc_us-gaap_RangeMember_5FFA828BE2B903BAD04708AAECC00A50_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_BD6BBBB43E7C55E790984223C603990D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_4A94A5360DE3576E8415C248130A0DE8" xlink:to="loc_us-gaap_RangeMember_BD6BBBB43E7C55E790984223C603990D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetsByMajorClassTextBlock" xlink:label="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetsByMajorClassTextBlock_583AAC958AB855EBA5D7EC2EAB167CD4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_73D3337CE2E2585D88960CFEC922834F" xlink:to="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetsByMajorClassTextBlock_583AAC958AB855EBA5D7EC2EAB167CD4" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/IncomeTaxes" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/IncomeTaxesDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/NoncontrollingInterestPuts" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/NoncontrollingInterestPutsDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_MinorityInterestLineItems" xlink:label="loc_us-gaap_MinorityInterestLineItems_BE9B81426B9D5189894BC332ADDDF2D9" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_MinorityInterestTable" xlink:label="loc_us-gaap_MinorityInterestTable_F8592131A6E15A6DA1939732CBDEDC34" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_MinorityInterestLineItems_BE9B81426B9D5189894BC332ADDDF2D9" xlink:to="loc_us-gaap_MinorityInterestTable_F8592131A6E15A6DA1939732CBDEDC34" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_E81DED9E7C855ABC9BC1AFF7CFD238C4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_MinorityInterestTable_F8592131A6E15A6DA1939732CBDEDC34" xlink:to="loc_dei_LegalEntityAxis_E81DED9E7C855ABC9BC1AFF7CFD238C4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_E6C439D0D2352971B635C940317FFEEB_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_E81DED9E7C855ABC9BC1AFF7CFD238C4" xlink:to="loc_dei_EntityDomain_E6C439D0D2352971B635C940317FFEEB_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_02CB36A96C1F5A52976B0AD0DA671834" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_E81DED9E7C855ABC9BC1AFF7CFD238C4" xlink:to="loc_dei_EntityDomain_02CB36A96C1F5A52976B0AD0DA671834" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/PensionAndPostretirementPlans" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/PensionAndPostretirementPlansDefinedContributionPlansDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_CF9CB382AD8DD4F5AE7FE24C229D5D8C" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_128E853B7D53EC813F92E24C229D3C5B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_CF9CB382AD8DD4F5AE7FE24C229D5D8C" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_128E853B7D53EC813F92E24C229D3C5B" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis" xlink:label="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_D88B9214F4E47B71A393E24C229D0736" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_128E853B7D53EC813F92E24C229D3C5B" xlink:to="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_D88B9214F4E47B71A393E24C229D0736" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlansDomain" xlink:label="loc_us-gaap_DefinedBenefitPlansDomain_FEEC6946F7305CEA109FE24C229DB8C2_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_D88B9214F4E47B71A393E24C229D0736" xlink:to="loc_us-gaap_DefinedBenefitPlansDomain_FEEC6946F7305CEA109FE24C229DB8C2_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlansDomain" xlink:label="loc_us-gaap_DefinedBenefitPlansDomain_FEEC6946F7305CEA109FE24C229DB8C2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_D88B9214F4E47B71A393E24C229D0736" xlink:to="loc_us-gaap_DefinedBenefitPlansDomain_FEEC6946F7305CEA109FE24C229DB8C2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPostretirementHealthCoverageMember" xlink:label="loc_us-gaap_DefinedBenefitPostretirementHealthCoverageMember_1B035E5D1BB2916EC1A3E24C229DBC9B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlansDomain_FEEC6946F7305CEA109FE24C229DB8C2" xlink:to="loc_us-gaap_DefinedBenefitPostretirementHealthCoverageMember_1B035E5D1BB2916EC1A3E24C229DBC9B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_2CF6E88C5A6EB50D75D1E24C229D022E" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_128E853B7D53EC813F92E24C229D3C5B" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_2CF6E88C5A6EB50D75D1E24C229D022E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_67584C96CBDE52EBB82CE24C229D4D2A_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_2CF6E88C5A6EB50D75D1E24C229D022E" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_67584C96CBDE52EBB82CE24C229D4D2A_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_67584C96CBDE52EBB82CE24C229D4D2A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_2CF6E88C5A6EB50D75D1E24C229D022E" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_67584C96CBDE52EBB82CE24C229D4D2A" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CovanceMember" xlink:label="loc_lh_CovanceMember_9B854AA5A889DBA01387E24C229D3C4F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_67584C96CBDE52EBB82CE24C229D4D2A" xlink:to="loc_lh_CovanceMember_9B854AA5A889DBA01387E24C229D3C4F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis" xlink:label="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_B4EBEE19DBF743615B91E24C229D6E6A" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_128E853B7D53EC813F92E24C229D3C5B" xlink:to="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_B4EBEE19DBF743615B91E24C229D6E6A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PlanAssetCategoriesDomain" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_CA2F0E5D230F7DC8BE2AE24C229DF707_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_B4EBEE19DBF743615B91E24C229D6E6A" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_CA2F0E5D230F7DC8BE2AE24C229DF707_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PlanAssetCategoriesDomain" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_CA2F0E5D230F7DC8BE2AE24C229DF707" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_B4EBEE19DBF743615B91E24C229D6E6A" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_CA2F0E5D230F7DC8BE2AE24C229DF707" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_FE03902E98A17FAACDC7E24C229DE1B8" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_128E853B7D53EC813F92E24C229D3C5B" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_FE03902E98A17FAACDC7E24C229DE1B8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_9501B8972D621EC5DB11E24C229DCA5B_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_FE03902E98A17FAACDC7E24C229DE1B8" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_9501B8972D621EC5DB11E24C229DCA5B_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_9501B8972D621EC5DB11E24C229DCA5B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_FE03902E98A17FAACDC7E24C229DE1B8" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_9501B8972D621EC5DB11E24C229DCA5B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost" xlink:label="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_5DE0CF2DDCD2456E6123E24C229D6E14" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_CF9CB382AD8DD4F5AE7FE24C229D5D8C" xlink:to="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_5DE0CF2DDCD2456E6123E24C229D6E14" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_NonElectiveContribution" xlink:label="loc_lh_NonElectiveContribution_669DA2921F59C820087FE24C229D7EA2" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_CF9CB382AD8DD4F5AE7FE24C229D5D8C" xlink:to="loc_lh_NonElectiveContribution_669DA2921F59C820087FE24C229D7EA2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedContributionPlanCostRecognized" xlink:label="loc_us-gaap_DefinedContributionPlanCostRecognized_9E3AA0C507BE2A99B9FEE24C229D6A62" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_CF9CB382AD8DD4F5AE7FE24C229D5D8C" xlink:to="loc_us-gaap_DefinedContributionPlanCostRecognized_9E3AA0C507BE2A99B9FEE24C229D6A62" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_DiscretionaryContributionPercentageMininum" xlink:label="loc_lh_DiscretionaryContributionPercentageMininum_DB941A7E42EDE2CCA087E24C229D0BFB" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_CF9CB382AD8DD4F5AE7FE24C229D5D8C" xlink:to="loc_lh_DiscretionaryContributionPercentageMininum_DB941A7E42EDE2CCA087E24C229D0BFB" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_DiscretionaryContributionPercentageMaximum" xlink:label="loc_lh_DiscretionaryContributionPercentageMaximum_5FEE641F4C564DBBA45CE24C229DD88B" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_CF9CB382AD8DD4F5AE7FE24C229D5D8C" xlink:to="loc_lh_DiscretionaryContributionPercentageMaximum_5FEE641F4C564DBBA45CE24C229DD88B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch" xlink:label="loc_us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch_F4EB754F13D656282A9BE24C72D5CCB7" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_CF9CB382AD8DD4F5AE7FE24C229D5D8C" xlink:to="loc_us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch_F4EB754F13D656282A9BE24C72D5CCB7" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/PensionAndPostretirementPlansDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_F762675AA8B2415465A4D8544A35AD76" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_54B44FA5FD225FC707BDD8544A343570" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_F762675AA8B2415465A4D8544A35AD76" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_54B44FA5FD225FC707BDD8544A343570" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_024F2DC02789F17A0941D8544A345484" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_54B44FA5FD225FC707BDD8544A343570" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_024F2DC02789F17A0941D8544A345484" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_19C49D65BE5D9886CD64D8544A345827_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_024F2DC02789F17A0941D8544A345484" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_19C49D65BE5D9886CD64D8544A345827_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_19C49D65BE5D9886CD64D8544A345827" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_024F2DC02789F17A0941D8544A345484" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_19C49D65BE5D9886CD64D8544A345827" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CovanceMember" xlink:label="loc_lh_CovanceMember_1CF3611077A6163B75FAD8544A34B148" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_19C49D65BE5D9886CD64D8544A345827" xlink:to="loc_lh_CovanceMember_1CF3611077A6163B75FAD8544A34B148" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis" xlink:label="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_B70E7004B596689EA438D8544A34D382" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_54B44FA5FD225FC707BDD8544A343570" xlink:to="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_B70E7004B596689EA438D8544A34D382" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlansDomain" xlink:label="loc_us-gaap_DefinedBenefitPlansDomain_B07FC397B3191B18E123D8544A3406C4_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_B70E7004B596689EA438D8544A34D382" xlink:to="loc_us-gaap_DefinedBenefitPlansDomain_B07FC397B3191B18E123D8544A3406C4_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlansDomain" xlink:label="loc_us-gaap_DefinedBenefitPlansDomain_B07FC397B3191B18E123D8544A3406C4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_B70E7004B596689EA438D8544A34D382" xlink:to="loc_us-gaap_DefinedBenefitPlansDomain_B07FC397B3191B18E123D8544A3406C4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PensionPlansDefinedBenefitMember" xlink:label="loc_us-gaap_PensionPlansDefinedBenefitMember_EA157625FAD2FA27616AD8544A340799" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlansDomain_B07FC397B3191B18E123D8544A3406C4" xlink:to="loc_us-gaap_PensionPlansDefinedBenefitMember_EA157625FAD2FA27616AD8544A340799" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPostretirementHealthCoverageMember" xlink:label="loc_us-gaap_DefinedBenefitPostretirementHealthCoverageMember_0933544A9134A20CBEBCD8544A342F5E" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlansDomain_B07FC397B3191B18E123D8544A3406C4" xlink:to="loc_us-gaap_DefinedBenefitPostretirementHealthCoverageMember_0933544A9134A20CBEBCD8544A342F5E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_OtherPensionPlansPostretirementOrSupplementalPlansDefinedBenefitMember" xlink:label="loc_us-gaap_OtherPensionPlansPostretirementOrSupplementalPlansDefinedBenefitMember_31A459523847F7125841D8544A35CFE0" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlansDomain_B07FC397B3191B18E123D8544A3406C4" xlink:to="loc_us-gaap_OtherPensionPlansPostretirementOrSupplementalPlansDefinedBenefitMember_31A459523847F7125841D8544A35CFE0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_OtherPensionPlansDefinedBenefitMember" xlink:label="loc_us-gaap_OtherPensionPlansDefinedBenefitMember_B47EB5E182F8422EB098D8544A35AEB1" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlansDomain_B07FC397B3191B18E123D8544A3406C4" xlink:to="loc_us-gaap_OtherPensionPlansDefinedBenefitMember_B47EB5E182F8422EB098D8544A35AEB1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember" xlink:label="loc_us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember_7B194E6FA7CFF2F68AF3D8544A353678" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlansDomain_B07FC397B3191B18E123D8544A3406C4" xlink:to="loc_us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember_7B194E6FA7CFF2F68AF3D8544A353678" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember" xlink:label="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_CF3F353740C34F30A4EED8544A3525E3" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlansDomain_B07FC397B3191B18E123D8544A3406C4" xlink:to="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_CF3F353740C34F30A4EED8544A3525E3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PensionContributions" xlink:label="loc_us-gaap_PensionContributions_215B31BE2DF080D5EF32D8544A354120" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_F762675AA8B2415465A4D8544A35AD76" xlink:to="loc_us-gaap_PensionContributions_215B31BE2DF080D5EF32D8544A354120" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlanBenefitObligation" xlink:label="loc_us-gaap_DefinedBenefitPlanBenefitObligation_F20141674AB1021BF196D8544A35694D" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_F762675AA8B2415465A4D8544A35AD76" xlink:to="loc_us-gaap_DefinedBenefitPlanBenefitObligation_F20141674AB1021BF196D8544A35694D" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_NonElectiveContribution" xlink:label="loc_lh_NonElectiveContribution_109DB206C44DF9711BCFD8544A35148E" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_F762675AA8B2415465A4D8544A35AD76" xlink:to="loc_lh_NonElectiveContribution_109DB206C44DF9711BCFD8544A35148E" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_DiscretionaryContributionPercentageMininum" xlink:label="loc_lh_DiscretionaryContributionPercentageMininum_58BAC96E1ED147D6F923D8544A3558B7" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_F762675AA8B2415465A4D8544A35AD76" xlink:to="loc_lh_DiscretionaryContributionPercentageMininum_58BAC96E1ED147D6F923D8544A3558B7" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_DiscretionaryContributionPercentageMaximum" xlink:label="loc_lh_DiscretionaryContributionPercentageMaximum_DBDEFD510292993E6ABCD8544A35AA44" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_F762675AA8B2415465A4D8544A35AD76" xlink:to="loc_lh_DiscretionaryContributionPercentageMaximum_DBDEFD510292993E6ABCD8544A35AA44" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedContributionPlanCostRecognized" xlink:label="loc_us-gaap_DefinedContributionPlanCostRecognized_AA4D24837A94A288CA7AD8544A35A6BE" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_F762675AA8B2415465A4D8544A35AD76" xlink:to="loc_us-gaap_DefinedContributionPlanCostRecognized_AA4D24837A94A288CA7AD8544A35A6BE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPensionPlansAndDefinedBenefitPostretirementPlansDisclosureAbstract" xlink:label="loc_us-gaap_DefinedBenefitPensionPlansAndDefinedBenefitPostretirementPlansDisclosureAbstract_2E7FBBE732B73117325DD8544A356983" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_F762675AA8B2415465A4D8544A35AD76" xlink:to="loc_us-gaap_DefinedBenefitPensionPlansAndDefinedBenefitPostretirementPlansDisclosureAbstract_2E7FBBE732B73117325DD8544A356983" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlanServiceCost" xlink:label="loc_us-gaap_DefinedBenefitPlanServiceCost_16C61036DB960BD7E692D8544A35DED7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPensionPlansAndDefinedBenefitPostretirementPlansDisclosureAbstract_2E7FBBE732B73117325DD8544A356983" xlink:to="loc_us-gaap_DefinedBenefitPlanServiceCost_16C61036DB960BD7E692D8544A35DED7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlanInterestCost" xlink:label="loc_us-gaap_DefinedBenefitPlanInterestCost_5675BB62C1BE112F800ED8544A359CA6" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPensionPlansAndDefinedBenefitPostretirementPlansDisclosureAbstract_2E7FBBE732B73117325DD8544A356983" xlink:to="loc_us-gaap_DefinedBenefitPlanInterestCost_5675BB62C1BE112F800ED8544A359CA6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets_45460B2976F890B04958D8544A353208" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPensionPlansAndDefinedBenefitPostretirementPlansDisclosureAbstract_2E7FBBE732B73117325DD8544A356983" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets_45460B2976F890B04958D8544A353208" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_NetDefinedBenefitPlanAmortizationAndDeferral" xlink:label="loc_lh_NetDefinedBenefitPlanAmortizationAndDeferral_95FF22C1F66E047D7AFED8544A35B612" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPensionPlansAndDefinedBenefitPostretirementPlansDisclosureAbstract_2E7FBBE732B73117325DD8544A356983" xlink:to="loc_lh_NetDefinedBenefitPlanAmortizationAndDeferral_95FF22C1F66E047D7AFED8544A35B612" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost" xlink:label="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_F4CBF9E68299AA941F20D8544A36BBE7" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPensionPlansAndDefinedBenefitPostretirementPlansDisclosureAbstract_2E7FBBE732B73117325DD8544A356983" xlink:to="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_F4CBF9E68299AA941F20D8544A36BBE7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlanOtherChanges" xlink:label="loc_us-gaap_DefinedBenefitPlanOtherChanges_16084B6FCF184B343FADD8544A3620B4" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_F762675AA8B2415465A4D8544A35AD76" xlink:to="loc_us-gaap_DefinedBenefitPlanOtherChanges_16084B6FCF184B343FADD8544A3620B4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate_781C7A1F73F9D338CE79D8544A369F93" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_F762675AA8B2415465A4D8544A35AD76" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate_781C7A1F73F9D338CE79D8544A369F93" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_DefinedBenefitPlanExpectedRateOfReturnOnAssetsOtherAssets" xlink:label="loc_lh_DefinedBenefitPlanExpectedRateOfReturnOnAssetsOtherAssets_B6F52D9FCA92347E3AE7D8544A3623C0" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_F762675AA8B2415465A4D8544A35AD76" xlink:to="loc_lh_DefinedBenefitPlanExpectedRateOfReturnOnAssetsOtherAssets_B6F52D9FCA92347E3AE7D8544A3623C0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease_3A45710D60C24C83B349D8544A362B91" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_F762675AA8B2415465A4D8544A35AD76" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease_3A45710D60C24C83B349D8544A362B91" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToCurtailments" xlink:label="loc_us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToCurtailments_31BE74DE6F651311D984D8544A369CEE" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_F762675AA8B2415465A4D8544A35AD76" xlink:to="loc_us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToCurtailments_31BE74DE6F651311D984D8544A369CEE" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/PensionAndPostretirementPlansTables" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlansAndOtherPostretirementBenefitPlansTableTextBlockLineItems" xlink:label="loc_us-gaap_DefinedBenefitPlansAndOtherPostretirementBenefitPlansTableTextBlockLineItems_2C1D77F9003752E5A9F3910EC39964AB" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_61720A0B41DD5577BD474C45D3B78E26" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlansAndOtherPostretirementBenefitPlansTableTextBlockLineItems_2C1D77F9003752E5A9F3910EC39964AB" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_61720A0B41DD5577BD474C45D3B78E26" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis" xlink:label="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_48D4E25D8A9B5E9998DB129CEA3B2C6D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_61720A0B41DD5577BD474C45D3B78E26" xlink:to="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_48D4E25D8A9B5E9998DB129CEA3B2C6D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlansDomain" xlink:label="loc_us-gaap_DefinedBenefitPlansDomain_A26122F76E006299C016328C3AE5BC26_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_48D4E25D8A9B5E9998DB129CEA3B2C6D" xlink:to="loc_us-gaap_DefinedBenefitPlansDomain_A26122F76E006299C016328C3AE5BC26_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlansDomain" xlink:label="loc_us-gaap_DefinedBenefitPlansDomain_C68173B3C6BC577CBAA19284205AA18F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_48D4E25D8A9B5E9998DB129CEA3B2C6D" xlink:to="loc_us-gaap_DefinedBenefitPlansDomain_C68173B3C6BC577CBAA19284205AA18F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis" xlink:label="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_D74F6002863B597F8B07DC72052F2F33" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_61720A0B41DD5577BD474C45D3B78E26" xlink:to="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_D74F6002863B597F8B07DC72052F2F33" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PlanAssetCategoriesDomain" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_5E56ED26CEABEE0EC774328C3AE5895F_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_D74F6002863B597F8B07DC72052F2F33" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_5E56ED26CEABEE0EC774328C3AE5895F_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PlanAssetCategoriesDomain" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_14E7F7DECB6A550BB00B87CF3C9F9878" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_D74F6002863B597F8B07DC72052F2F33" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_14E7F7DECB6A550BB00B87CF3C9F9878" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedContributionPlanDisclosuresTableTextBlock" xlink:label="loc_us-gaap_DefinedContributionPlanDisclosuresTableTextBlock_7B44A3C450DD568294BE05DDFC905DF0" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlansAndOtherPostretirementBenefitPlansTableTextBlockLineItems_2C1D77F9003752E5A9F3910EC39964AB" xlink:to="loc_us-gaap_DefinedContributionPlanDisclosuresTableTextBlock_7B44A3C450DD568294BE05DDFC905DF0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfCostsOfRetirementPlansTableTextBlock" xlink:label="loc_us-gaap_ScheduleOfCostsOfRetirementPlansTableTextBlock_43B5F8634B7C5DBC801BD48FF9ADBF05" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlansAndOtherPostretirementBenefitPlansTableTextBlockLineItems_2C1D77F9003752E5A9F3910EC39964AB" xlink:to="loc_us-gaap_ScheduleOfCostsOfRetirementPlansTableTextBlock_43B5F8634B7C5DBC801BD48FF9ADBF05" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/PreferredStockAndCommonShareholdersEquity" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/PreferredStockAndCommonShareholdersEquityDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ClassOfStockLineItems" xlink:label="loc_us-gaap_ClassOfStockLineItems_A40C193F0E8F5435953C4452F4318154" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfStockByClassTable" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_C5FDB6C46BA25BE293E842AF57A9999A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_A40C193F0E8F5435953C4452F4318154" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_C5FDB6C46BA25BE293E842AF57A9999A" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_E85C4F097D94500FB04A1EFCB478DC8B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_C5FDB6C46BA25BE293E842AF57A9999A" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_E85C4F097D94500FB04A1EFCB478DC8B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_0E6EAC7899F0FCD11BAF090DFD4EB371_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_E85C4F097D94500FB04A1EFCB478DC8B" xlink:to="loc_us-gaap_EquityComponentDomain_0E6EAC7899F0FCD11BAF090DFD4EB371_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_726EE8D4410A50588D78964927EF7382" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_E85C4F097D94500FB04A1EFCB478DC8B" xlink:to="loc_us-gaap_EquityComponentDomain_726EE8D4410A50588D78964927EF7382" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_TreasuryStockMember" xlink:label="loc_us-gaap_TreasuryStockMember_3EEBA5A7B66054FFAA78BF0FE939154C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_726EE8D4410A50588D78964927EF7382" xlink:to="loc_us-gaap_TreasuryStockMember_3EEBA5A7B66054FFAA78BF0FE939154C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CommonStockMember" xlink:label="loc_us-gaap_CommonStockMember_79BCE35246D858E0B206F864DBAC48D7" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_726EE8D4410A50588D78964927EF7382" xlink:to="loc_us-gaap_CommonStockMember_79BCE35246D858E0B206F864DBAC48D7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="loc_us-gaap_CreditFacilityAxis_95522A9682F05521A16B268B403B1398" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_C5FDB6C46BA25BE293E842AF57A9999A" xlink:to="loc_us-gaap_CreditFacilityAxis_95522A9682F05521A16B268B403B1398" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_2D3DAB80FCF0189046FE08E12FAA9166_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_95522A9682F05521A16B268B403B1398" xlink:to="loc_us-gaap_CreditFacilityDomain_2D3DAB80FCF0189046FE08E12FAA9166_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_B6B87AB2A73D59B78340E60B7C66B4FF" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_95522A9682F05521A16B268B403B1398" xlink:to="loc_us-gaap_CreditFacilityDomain_B6B87AB2A73D59B78340E60B7C66B4FF" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_TargetRatiotoResumeShareRepurchaseMember" xlink:label="loc_lh_TargetRatiotoResumeShareRepurchaseMember_A0BB7F5A4F7B5CE29F419767E54922A8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_B6B87AB2A73D59B78340E60B7C66B4FF" xlink:to="loc_lh_TargetRatiotoResumeShareRepurchaseMember_A0BB7F5A4F7B5CE29F419767E54922A8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_CE95A8244D4A5FD989D86847140C3885" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_C5FDB6C46BA25BE293E842AF57A9999A" xlink:to="loc_us-gaap_StatementClassOfStockAxis_CE95A8244D4A5FD989D86847140C3885" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_47556E105A15E34BFDAF08E12FAAA692_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_CE95A8244D4A5FD989D86847140C3885" xlink:to="loc_us-gaap_ClassOfStockDomain_47556E105A15E34BFDAF08E12FAAA692_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_3E5A3D4D57735E168209134633E1E35F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_CE95A8244D4A5FD989D86847140C3885" xlink:to="loc_us-gaap_ClassOfStockDomain_3E5A3D4D57735E168209134633E1E35F" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_StockConversionRateForZeroCouponSubordinatedNotes" xlink:label="loc_lh_StockConversionRateForZeroCouponSubordinatedNotes_7A5D5FCAFBE2501ABFFC80D69870C0FD" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_A40C193F0E8F5435953C4452F4318154" xlink:to="loc_lh_StockConversionRateForZeroCouponSubordinatedNotes_7A5D5FCAFBE2501ABFFC80D69870C0FD" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_RestrictedStockAwardsAndPerformanceSharesSurrendered" xlink:label="loc_lh_RestrictedStockAwardsAndPerformanceSharesSurrendered_4A091F8BAD2C562F93157F1942F1AAF4" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_A40C193F0E8F5435953C4452F4318154" xlink:to="loc_lh_RestrictedStockAwardsAndPerformanceSharesSurrendered_4A091F8BAD2C562F93157F1942F1AAF4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardForfeited" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardForfeited_90FC7868804555DBBD7381BE11345A65" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_A40C193F0E8F5435953C4452F4318154" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardForfeited_90FC7868804555DBBD7381BE11345A65" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_397972ABDA495B128B4541F3D3ECB023" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_A40C193F0E8F5435953C4452F4318154" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_397972ABDA495B128B4541F3D3ECB023" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_DebtToEbitdaLeverageRatio" xlink:label="loc_lh_DebtToEbitdaLeverageRatio_017205DF04C05449A26D0EB4C25BDD66" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_A40C193F0E8F5435953C4452F4318154" xlink:to="loc_lh_DebtToEbitdaLeverageRatio_017205DF04C05449A26D0EB4C25BDD66" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_528D6D47F04E5C6895E7C6A2CAA28E77" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_A40C193F0E8F5435953C4452F4318154" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_528D6D47F04E5C6895E7C6A2CAA28E77" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PreferredStockSharesAuthorized" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_CB94683C8B5858C39E5055F815849533" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_A40C193F0E8F5435953C4452F4318154" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_CB94683C8B5858C39E5055F815849533" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PreferredStockParOrStatedValuePerShare" xlink:label="loc_us-gaap_PreferredStockParOrStatedValuePerShare_546840FBFF6152AEAE35FF6C48C007D2" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_A40C193F0E8F5435953C4452F4318154" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_546840FBFF6152AEAE35FF6C48C007D2" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/PreferredStockAndCommonShareholdersEquityPreferredStockAndCommonShareholdersEquityAccumulatedOtherComprehensiveEarningsDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/PreferredStockAndCommonShareholdersEquityTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/RestructuringAndOtherSpecialCharges" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/RestructuringAndOtherSpecialChargesDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringCostAndReserveLineItems" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_2FB0E1D4CD63595FC3E6FD4A3B8177A1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_2FB0E1D4CD63595FC3E6FD4A3B8177A1" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_52A24EE55B28649FC255FD4A3B81F95A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_2FB0E1D4CD63595FC3E6FD4A3B8177A1" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_52A24EE55B28649FC255FD4A3B81F95A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_211E6BB690CBD7DF7380FD4A3B81B5EE_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_52A24EE55B28649FC255FD4A3B81F95A" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_211E6BB690CBD7DF7380FD4A3B81B5EE_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_211E6BB690CBD7DF7380FD4A3B81B5EE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_52A24EE55B28649FC255FD4A3B81F95A" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_211E6BB690CBD7DF7380FD4A3B81B5EE" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CovanceMember" xlink:label="loc_lh_CovanceMember_71B1AEAEEC8710D44320FD4A3B8134F3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_211E6BB690CBD7DF7380FD4A3B81B5EE" xlink:to="loc_lh_CovanceMember_71B1AEAEEC8710D44320FD4A3B8134F3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_75AA346D45D3851C5BC8FD4A3B817212" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_2FB0E1D4CD63595FC3E6FD4A3B8177A1" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_75AA346D45D3851C5BC8FD4A3B817212" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_68E95F83269C16B6603EFD4A3B81A957_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_75AA346D45D3851C5BC8FD4A3B817212" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_68E95F83269C16B6603EFD4A3B81A957_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_68E95F83269C16B6603EFD4A3B81A957" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_75AA346D45D3851C5BC8FD4A3B817212" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_68E95F83269C16B6603EFD4A3B81A957" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_AC86C92433FAC6C1FAF4FD4A3B81AAB8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_68E95F83269C16B6603EFD4A3B81A957" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_AC86C92433FAC6C1FAF4FD4A3B81AAB8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_InterestExpenseMember" xlink:label="loc_us-gaap_InterestExpenseMember_F1D4B918693A5C44F021FD4A3B81B6F5" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_68E95F83269C16B6603EFD4A3B81A957" xlink:to="loc_us-gaap_InterestExpenseMember_F1D4B918693A5C44F021FD4A3B81B6F5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_298CBE6DCCE242DDB763FD4A3B8B3361" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_2FB0E1D4CD63595FC3E6FD4A3B8177A1" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_298CBE6DCCE242DDB763FD4A3B8B3361" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_9EBB404F1BD4C96B4E79FD4A3B8B7F6D_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_298CBE6DCCE242DDB763FD4A3B8B3361" xlink:to="loc_us-gaap_SegmentDomain_9EBB404F1BD4C96B4E79FD4A3B8B7F6D_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_9EBB404F1BD4C96B4E79FD4A3B8B7F6D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_298CBE6DCCE242DDB763FD4A3B8B3361" xlink:to="loc_us-gaap_SegmentDomain_9EBB404F1BD4C96B4E79FD4A3B8B7F6D" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CovanceDrugDevelopmentMember" xlink:label="loc_lh_CovanceDrugDevelopmentMember_13EAA1D46613F8475E54FD4A3B8B2EAE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_9EBB404F1BD4C96B4E79FD4A3B8B7F6D" xlink:to="loc_lh_CovanceDrugDevelopmentMember_13EAA1D46613F8475E54FD4A3B8B2EAE" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_LabCorpDiagnosticsMember" xlink:label="loc_lh_LabCorpDiagnosticsMember_A89E235296048B2EA1F4FD4A3B8B3233" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_9EBB404F1BD4C96B4E79FD4A3B8B7F6D" xlink:to="loc_lh_LabCorpDiagnosticsMember_A89E235296048B2EA1F4FD4A3B8B3233" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringCostAndReserveAxis" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_06F9BFB5EE9B647928CBFD4A3B8B02AC" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_2FB0E1D4CD63595FC3E6FD4A3B8177A1" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_06F9BFB5EE9B647928CBFD4A3B8B02AC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_556B775FB7AEC60009FEFD4A3B8BFB6F_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_06F9BFB5EE9B647928CBFD4A3B8B02AC" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_556B775FB7AEC60009FEFD4A3B8BFB6F_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_556B775FB7AEC60009FEFD4A3B8BFB6F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_06F9BFB5EE9B647928CBFD4A3B8B02AC" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_556B775FB7AEC60009FEFD4A3B8BFB6F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_EmployeeSeveranceMember" xlink:label="loc_us-gaap_EmployeeSeveranceMember_BA842B345B374BF07807FD4A3B8B9628" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_556B775FB7AEC60009FEFD4A3B8BFB6F" xlink:to="loc_us-gaap_EmployeeSeveranceMember_BA842B345B374BF07807FD4A3B8B9628" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FacilityClosingMember" xlink:label="loc_us-gaap_FacilityClosingMember_09439B9D57C44CBA7DA5FD4A3B8BF07D" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_556B775FB7AEC60009FEFD4A3B8BFB6F" xlink:to="loc_us-gaap_FacilityClosingMember_09439B9D57C44CBA7DA5FD4A3B8BF07D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_GoodwillAcquiredDuringPeriod" xlink:label="loc_us-gaap_GoodwillAcquiredDuringPeriod_75C4C374506B0E0E4A2EFD4A3B8B960E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_us-gaap_GoodwillAcquiredDuringPeriod_75C4C374506B0E0E4A2EFD4A3B8B960E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringReserve" xlink:label="loc_us-gaap_RestructuringReserve_EE9606A9E095BAA54C54FD4A3B8B4641" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_us-gaap_RestructuringReserve_EE9606A9E095BAA54C54FD4A3B8B4641" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringCharges" xlink:label="loc_us-gaap_RestructuringCharges_14DF554885323DC9F16FFD4A3B8BDA1B" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_us-gaap_RestructuringCharges_14DF554885323DC9F16FFD4A3B8BDA1B" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_RestructuringReserveSettledWithCashAndOtherAdjustment" xlink:label="loc_lh_RestructuringReserveSettledWithCashAndOtherAdjustment_ED3D4091F9947E7AB5EFFD4A3B8B165E" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_lh_RestructuringReserveSettledWithCashAndOtherAdjustment_ED3D4091F9947E7AB5EFFD4A3B8B165E" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_NetRestructuringCharges" xlink:label="loc_lh_NetRestructuringCharges_CA79E6107983C7536F82FD4A3B8B48B7" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_lh_NetRestructuringCharges_CA79E6107983C7536F82FD4A3B8B48B7" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_OtherSpecialCharges" xlink:label="loc_lh_OtherSpecialCharges_D2031A9446BEF1AB0D47FD4A3B8B5A52" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_lh_OtherSpecialCharges_D2031A9446BEF1AB0D47FD4A3B8B5A52" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Specialchargeonetimetaxliability" xlink:label="loc_lh_Specialchargeonetimetaxliability_5096AFE14D58613C216AFD4A3B8BF6BC" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_lh_Specialchargeonetimetaxliability_5096AFE14D58613C216AFD4A3B8BF6BC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_AmortizationOfDebtDiscountPremium" xlink:label="loc_us-gaap_AmortizationOfDebtDiscountPremium_6D26EC0BE9F9914480B1FD4A3B8BF850" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_us-gaap_AmortizationOfDebtDiscountPremium_6D26EC0BE9F9914480B1FD4A3B8BF850" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SeveranceCosts1" xlink:label="loc_us-gaap_SeveranceCosts1_60B9169CCB008130BCD5FD4A3B8BF7AC" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_us-gaap_SeveranceCosts1_60B9169CCB008130BCD5FD4A3B8BF7AC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessExitCosts1" xlink:label="loc_us-gaap_BusinessExitCosts1_4B98A887160A15BDFA56FD4A3B8B726B" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_us-gaap_BusinessExitCosts1_4B98A887160A15BDFA56FD4A3B8B726B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringReserveAccrualAdjustment1" xlink:label="loc_us-gaap_RestructuringReserveAccrualAdjustment1_3429122E791C8171A201FD4A3B8BADC1" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_us-gaap_RestructuringReserveAccrualAdjustment1_3429122E791C8171A201FD4A3B8BADC1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringReserveCurrent" xlink:label="loc_us-gaap_RestructuringReserveCurrent_C60B91772D2E90CC0D00FD4A3B8BB056" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_us-gaap_RestructuringReserveCurrent_C60B91772D2E90CC0D00FD4A3B8BB056" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringReserveNoncurrent" xlink:label="loc_us-gaap_RestructuringReserveNoncurrent_44CBEB87CB6ECBDED49AFD4A3B8BD29F" xlink:type="locator" />
    <definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_us-gaap_RestructuringReserveNoncurrent_44CBEB87CB6ECBDED49AFD4A3B8BD29F" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_FacilityRelatedRestructuringReserveAccrualAdjustment" xlink:label="loc_lh_FacilityRelatedRestructuringReserveAccrualAdjustment_438C4C81D3754FEC5730FD4A3B8BAB70" xlink:type="locator" />
    <definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_lh_FacilityRelatedRestructuringReserveAccrualAdjustment_438C4C81D3754FEC5730FD4A3B8BAB70" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Unusedfacilityrestructuringreserves" xlink:label="loc_lh_Unusedfacilityrestructuringreserves_E696C12916D8EAD5EB25FD4A3B8BDB29" xlink:type="locator" />
    <definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_lh_Unusedfacilityrestructuringreserves_E696C12916D8EAD5EB25FD4A3B8BDB29" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Specialchargewinddownofminvolumecontract" xlink:label="loc_lh_Specialchargewinddownofminvolumecontract_FABF36E91F8C93A3D99FFD4A3B8BB1FB" xlink:type="locator" />
    <definitionArc order="16" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_lh_Specialchargewinddownofminvolumecontract_FABF36E91F8C93A3D99FFD4A3B8BB1FB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessCombinationAcquisitionRelatedCosts" xlink:label="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_6C1E6A7162D787F17C02FD4C177EFA48" xlink:type="locator" />
    <definitionArc order="17" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_6C1E6A7162D787F17C02FD4C177EFA48" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_SpecialchargeSTequityretentionarrangement" xlink:label="loc_lh_SpecialchargeSTequityretentionarrangement_CB9C1AADCF073F5BD439FD4A3B8BCDD5" xlink:type="locator" />
    <definitionArc order="18" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_lh_SpecialchargeSTequityretentionarrangement_CB9C1AADCF073F5BD439FD4A3B8BCDD5" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Specialchargeacceleratedequitycompensation" xlink:label="loc_lh_Specialchargeacceleratedequitycompensation_F53EF98236CC6DF01A44FD4A3B8B3E2C" xlink:type="locator" />
    <definitionArc order="19" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_lh_Specialchargeacceleratedequitycompensation_F53EF98236CC6DF01A44FD4A3B8B3E2C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionCostOfAcquiredEntityTransactionCosts" xlink:label="loc_us-gaap_BusinessAcquisitionCostOfAcquiredEntityTransactionCosts_E1AB4D575B1C5D722C41FD4A3B8B8907" xlink:type="locator" />
    <definitionArc order="20" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_us-gaap_BusinessAcquisitionCostOfAcquiredEntityTransactionCosts_E1AB4D575B1C5D722C41FD4A3B8B8907" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Noncashlossondissolutionofequityinvestment" xlink:label="loc_lh_Noncashlossondissolutionofequityinvestment_AEC488273A20CCE42F76FD4A3B8BDCAE" xlink:type="locator" />
    <definitionArc order="21" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_lh_Noncashlossondissolutionofequityinvestment_AEC488273A20CCE42F76FD4A3B8BDCAE" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Specialchargeconsultingexpense" xlink:label="loc_lh_Specialchargeconsultingexpense_2AE4D76FBAC03F411014FD4A3B8B483E" xlink:type="locator" />
    <definitionArc order="22" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_B4DB76B71FD9E3B050D5FD4A3B8B9428" xlink:to="loc_lh_Specialchargeconsultingexpense_2AE4D76FBAC03F411014FD4A3B8B483E" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/RestructuringAndOtherSpecialChargesRestructuringAndOtherSpecialChargesDetailTables" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringCostAndReserveLineItems" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_2A6D6FF96D4B56CDAA79B1F7DF8026FF" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_C8EAEEF944425E4C8D612AD900A6C7C1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_2A6D6FF96D4B56CDAA79B1F7DF8026FF" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_C8EAEEF944425E4C8D612AD900A6C7C1" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConsolidationItemsAxis" xlink:label="loc_us-gaap_ConsolidationItemsAxis_07DBEE5C747C53AC991CB27A54263B26" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_C8EAEEF944425E4C8D612AD900A6C7C1" xlink:to="loc_us-gaap_ConsolidationItemsAxis_07DBEE5C747C53AC991CB27A54263B26" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConsolidationItemsDomain" xlink:label="loc_us-gaap_ConsolidationItemsDomain_AE2F68BCB9E86FEE927008AAECDE9A0B_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConsolidationItemsAxis_07DBEE5C747C53AC991CB27A54263B26" xlink:to="loc_us-gaap_ConsolidationItemsDomain_AE2F68BCB9E86FEE927008AAECDE9A0B_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConsolidationItemsDomain" xlink:label="loc_us-gaap_ConsolidationItemsDomain_047D925A6EC65F44B57315B541E018FE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConsolidationItemsAxis_07DBEE5C747C53AC991CB27A54263B26" xlink:to="loc_us-gaap_ConsolidationItemsDomain_047D925A6EC65F44B57315B541E018FE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_54DA200B982057C38E2343757D592490" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_C8EAEEF944425E4C8D612AD900A6C7C1" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_54DA200B982057C38E2343757D592490" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_A651283E1EB95D69529108AAECDE4876_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_54DA200B982057C38E2343757D592490" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_A651283E1EB95D69529108AAECDE4876_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0A896A21A85B5D369437ABB359FF43EA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_54DA200B982057C38E2343757D592490" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0A896A21A85B5D369437ABB359FF43EA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringCostAndReserveAxis" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_A9B5922F7B375ACB84052CB53AC9056A" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_C8EAEEF944425E4C8D612AD900A6C7C1" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_A9B5922F7B375ACB84052CB53AC9056A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_A0D5D60F95B089720AC908AAECDE191B_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_A9B5922F7B375ACB84052CB53AC9056A" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_A0D5D60F95B089720AC908AAECDE191B_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_664D1E691B99522F9332842C2EF57416" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_A9B5922F7B375ACB84052CB53AC9056A" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_664D1E691B99522F9332842C2EF57416" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringPlanAxis" xlink:label="loc_us-gaap_RestructuringPlanAxis_86BF1ECCDBDE58538B63C4E7CCAC31A6" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_C8EAEEF944425E4C8D612AD900A6C7C1" xlink:to="loc_us-gaap_RestructuringPlanAxis_86BF1ECCDBDE58538B63C4E7CCAC31A6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringPlanDomain" xlink:label="loc_us-gaap_RestructuringPlanDomain_AE82F2CCC693435CCC8F08AAECDE6FE0_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringPlanAxis_86BF1ECCDBDE58538B63C4E7CCAC31A6" xlink:to="loc_us-gaap_RestructuringPlanDomain_AE82F2CCC693435CCC8F08AAECDE6FE0_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringPlanDomain" xlink:label="loc_us-gaap_RestructuringPlanDomain_FAA67DC43DED5A3D933E5A6CD2E2478B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringPlanAxis_86BF1ECCDBDE58538B63C4E7CCAC31A6" xlink:to="loc_us-gaap_RestructuringPlanDomain_FAA67DC43DED5A3D933E5A6CD2E2478B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_621779926828593EB4DAEC5D8B57B065" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_C8EAEEF944425E4C8D612AD900A6C7C1" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_621779926828593EB4DAEC5D8B57B065" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_8808BEB4F69A1AD9AD1D08AAECDED8A4_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_621779926828593EB4DAEC5D8B57B065" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_8808BEB4F69A1AD9AD1D08AAECDED8A4_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_8A6C73F90CEA5A7388557BDDD45CE7C7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_621779926828593EB4DAEC5D8B57B065" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_8A6C73F90CEA5A7388557BDDD45CE7C7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_OtherComprehensiveIncomeLocationAxis" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLocationAxis_0D57E5ED267457218B00F2F2901B979B" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_C8EAEEF944425E4C8D612AD900A6C7C1" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLocationAxis_0D57E5ED267457218B00F2F2901B979B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_OtherComprehensiveIncomeLocationDomain" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLocationDomain_72481419BB4A832C021008AAECDE85AD_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLocationAxis_0D57E5ED267457218B00F2F2901B979B" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLocationDomain_72481419BB4A832C021008AAECDE85AD_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_OtherComprehensiveIncomeLocationDomain" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLocationDomain_C79993691B0255D7B7D9C2C698341E1C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLocationAxis_0D57E5ED267457218B00F2F2901B979B" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLocationDomain_C79993691B0255D7B7D9C2C698341E1C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfRestructuringReserveByTypeOfCostTextBlock" xlink:label="loc_us-gaap_ScheduleOfRestructuringReserveByTypeOfCostTextBlock_4CC1935EAE5B5263911D59E24BF2657D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_2A6D6FF96D4B56CDAA79B1F7DF8026FF" xlink:to="loc_us-gaap_ScheduleOfRestructuringReserveByTypeOfCostTextBlock_4CC1935EAE5B5263911D59E24BF2657D" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/RestructuringReserves" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/RestructuringReservesDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringCostAndReserveLineItems" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_988F3F3D142659F8A3D0AADAF8CFF8F7" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_EA8EC08C3BB05F52B555C34E31229E68" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_988F3F3D142659F8A3D0AADAF8CFF8F7" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_EA8EC08C3BB05F52B555C34E31229E68" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_660DF61A10EA5F5690272920FAFE7C32" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_EA8EC08C3BB05F52B555C34E31229E68" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_660DF61A10EA5F5690272920FAFE7C32" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_319C46B49962999D4E59C940325B8D9F_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_660DF61A10EA5F5690272920FAFE7C32" xlink:to="loc_us-gaap_SegmentDomain_319C46B49962999D4E59C940325B8D9F_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_788BF51B828F5890A8AC27506A35226C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_660DF61A10EA5F5690272920FAFE7C32" xlink:to="loc_us-gaap_SegmentDomain_788BF51B828F5890A8AC27506A35226C" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_LabCorpDiagnosticsMember" xlink:label="loc_lh_LabCorpDiagnosticsMember_40072CC1BC3155C0B35E8F3A7D2FE3D0" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_788BF51B828F5890A8AC27506A35226C" xlink:to="loc_lh_LabCorpDiagnosticsMember_40072CC1BC3155C0B35E8F3A7D2FE3D0" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_CovanceDrugDevelopmentMember" xlink:label="loc_lh_CovanceDrugDevelopmentMember_661871686FE25BCF8A945377BBE11EC2" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_788BF51B828F5890A8AC27506A35226C" xlink:to="loc_lh_CovanceDrugDevelopmentMember_661871686FE25BCF8A945377BBE11EC2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_05578098825B5FE984337673760C9012" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_EA8EC08C3BB05F52B555C34E31229E68" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_05578098825B5FE984337673760C9012" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_A9F6B258B89A28CAA809C940325BCD0E_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_05578098825B5FE984337673760C9012" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_A9F6B258B89A28CAA809C940325BCD0E_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_1D4B0DC0ACD65660968B1B36EE0816AD" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_05578098825B5FE984337673760C9012" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_1D4B0DC0ACD65660968B1B36EE0816AD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_3ED66778E09F5A27A677FE0E52CD378D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_1D4B0DC0ACD65660968B1B36EE0816AD" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_3ED66778E09F5A27A677FE0E52CD378D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringCostAndReserveAxis" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_07D9E83AEE5F5141B7A59A2BAA71AC7E" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_EA8EC08C3BB05F52B555C34E31229E68" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_07D9E83AEE5F5141B7A59A2BAA71AC7E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_421964EA181FB0F77767C940325B70CB_24578904EA675A2EA5BB1A232B07967D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_07D9E83AEE5F5141B7A59A2BAA71AC7E" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_421964EA181FB0F77767C940325B70CB_24578904EA675A2EA5BB1A232B07967D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_02F3FAFCBC685189B7E0FF1E4F56DBF7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_07D9E83AEE5F5141B7A59A2BAA71AC7E" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_02F3FAFCBC685189B7E0FF1E4F56DBF7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_EmployeeSeveranceMember" xlink:label="loc_us-gaap_EmployeeSeveranceMember_A868D52B00835EE89984DDB8EB9CABFB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_02F3FAFCBC685189B7E0FF1E4F56DBF7" xlink:to="loc_us-gaap_EmployeeSeveranceMember_A868D52B00835EE89984DDB8EB9CABFB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FacilityClosingMember" xlink:label="loc_us-gaap_FacilityClosingMember_A2C6A611530F51ABB47EF97CC3C24CCD" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_02F3FAFCBC685189B7E0FF1E4F56DBF7" xlink:to="loc_us-gaap_FacilityClosingMember_A2C6A611530F51ABB47EF97CC3C24CCD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringReserve" xlink:label="loc_us-gaap_RestructuringReserve_4BB43D3710155D5A84FABB7D3AB4535E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_988F3F3D142659F8A3D0AADAF8CFF8F7" xlink:to="loc_us-gaap_RestructuringReserve_4BB43D3710155D5A84FABB7D3AB4535E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringCharges" xlink:label="loc_us-gaap_RestructuringCharges_E5BADF7E77F357EEAF3D3BFCCDCE3EE8" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_988F3F3D142659F8A3D0AADAF8CFF8F7" xlink:to="loc_us-gaap_RestructuringCharges_E5BADF7E77F357EEAF3D3BFCCDCE3EE8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringReserveAccrualAdjustment1" xlink:label="loc_us-gaap_RestructuringReserveAccrualAdjustment1_E64A16E8F35558FC91A2F799D2D51B73" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_988F3F3D142659F8A3D0AADAF8CFF8F7" xlink:to="loc_us-gaap_RestructuringReserveAccrualAdjustment1_E64A16E8F35558FC91A2F799D2D51B73" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_RestructuringReserveSettledWithCashAndOtherAdjustment" xlink:label="loc_lh_RestructuringReserveSettledWithCashAndOtherAdjustment_6AC52AE08636507EA0AB37655E31E040" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_988F3F3D142659F8A3D0AADAF8CFF8F7" xlink:to="loc_lh_RestructuringReserveSettledWithCashAndOtherAdjustment_6AC52AE08636507EA0AB37655E31E040" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringReserveCurrent" xlink:label="loc_us-gaap_RestructuringReserveCurrent_5D5D9DEBC8485C828BF80CD33EA466F5" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_988F3F3D142659F8A3D0AADAF8CFF8F7" xlink:to="loc_us-gaap_RestructuringReserveCurrent_5D5D9DEBC8485C828BF80CD33EA466F5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringReserveNoncurrent" xlink:label="loc_us-gaap_RestructuringReserveNoncurrent_864980996833519CBA6BF9CD3203DC12" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_988F3F3D142659F8A3D0AADAF8CFF8F7" xlink:to="loc_us-gaap_RestructuringReserveNoncurrent_864980996833519CBA6BF9CD3203DC12" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/SupplementalCashFlowInformation" xlink:type="extended" />
  <definitionLink xlink:role="http://www.labcorp.com/role/SupplementalCashFlowInformationDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtConversionLineItems" xlink:label="loc_us-gaap_DebtConversionLineItems_863BDDAD896E32B95719D86DE3E8F06E" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtConversionTable" xlink:label="loc_us-gaap_DebtConversionTable_60270AE544EFED5D0652D86DE3E7A347" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtConversionLineItems_863BDDAD896E32B95719D86DE3E8F06E" xlink:to="loc_us-gaap_DebtConversionTable_60270AE544EFED5D0652D86DE3E7A347" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentAxis" xlink:label="loc_us-gaap_DebtInstrumentAxis_AA79CB3F4676B8A01451D86DE3E8B89F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtConversionTable_60270AE544EFED5D0652D86DE3E7A347" xlink:to="loc_us-gaap_DebtInstrumentAxis_AA79CB3F4676B8A01451D86DE3E8B89F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_0BDD091E847232B19D28D86DE3E8658F_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_AA79CB3F4676B8A01451D86DE3E8B89F" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_0BDD091E847232B19D28D86DE3E8658F_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_0BDD091E847232B19D28D86DE3E8658F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_AA79CB3F4676B8A01451D86DE3E8B89F" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_0BDD091E847232B19D28D86DE3E8658F" xlink:type="arc" />
    <loc xlink:href="lh-20160930.xsd#lh_Noncashconversionofzerocouponconvertibledebt" xlink:label="loc_lh_Noncashconversionofzerocouponconvertibledebt_322E665ECB1219C212DCD86DE3E8CBB2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtConversionLineItems_863BDDAD896E32B95719D86DE3E8F06E" xlink:to="loc_lh_Noncashconversionofzerocouponconvertibledebt_322E665ECB1219C212DCD86DE3E8CBB2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueOfAssetsAcquired" xlink:label="loc_us-gaap_FairValueOfAssetsAcquired_8CD845A4515BE535A5DAD86DE3ED89AC" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtConversionLineItems_863BDDAD896E32B95719D86DE3E8F06E" xlink:to="loc_us-gaap_FairValueOfAssetsAcquired_8CD845A4515BE535A5DAD86DE3ED89AC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CapitalExpendituresIncurredButNotYetPaid" xlink:label="loc_us-gaap_CapitalExpendituresIncurredButNotYetPaid_DE3CF6E8BE1600DF6FC1D86DE3EDF78F" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtConversionLineItems_863BDDAD896E32B95719D86DE3E8F06E" xlink:to="loc_us-gaap_CapitalExpendituresIncurredButNotYetPaid_DE3CF6E8BE1600DF6FC1D86DE3EDF78F" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.labcorp.com/role/SupplementalCashFlowInformationTables" xlink:type="extended" />
</linkbase>
</XBRL>
