<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0101</schemaVersion>

    <documentType>4</documentType><periodOfReport>2003-06-30</periodOfReport>
    <issuer>
        <issuerCik>0000706539</issuerCik>
        <issuerName>CHIRON CORP</issuerName>
        <issuerTradingSymbol>CHIR</issuerTradingSymbol></issuer><reportingOwner><reportingOwnerId>
            <rptOwnerCik>0001216504</rptOwnerCik>
            <rptOwnerName>COLEMAN LEWIS W</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>  </rptOwnerStreet1>
            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>1</isDirector><isOfficer>0</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther></reportingOwnerRelationship></reportingOwner><nonDerivativeSecurity><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>2003-06-30</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>3654</value><footnoteId id="F1"/></transactionShares><transactionValue><value>0</value></transactionValue><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>12459</value><footnoteId id="F2"/><footnoteId id="F3"/></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value></value></natureOfOwnership></ownershipNature></nonDerivativeSecurity><footnotes><footnote id="F1">Represents grant of annual share rights (which are fully vested) made to non-employee directors of Issuer, which shall entitle the Reporting Person to receive that number of shares of Common Stock following cessation of service on Issuer's Board.</footnote><footnote id="F2">This amount includes 549 share rights which are subject to certain vesting.</footnote><footnote id="F3">Includes 95,466 shares held in a family trust, the Lewis W. Coleman and Susan G. Coleman Family Trust uta dated November 29, 1995, of which Reporting Person and his wife, are the trustees.</footnote></footnotes><ownerSignature><signatureName>Lewis W. Coleman, By Carolyn Mar-Andreotti as Atty-in-Fact</signatureName><signatureDate>2003-07-02</signatureDate></ownerSignature></ownershipDocument>

