<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-03-02</periodOfReport>

    <notSubjectToSection16>1</notSubjectToSection16>

    <issuer>
        <issuerCik>0000790705</issuerCik>
        <issuerName>TEKELEC</issuerName>
        <issuerTradingSymbol>TKLC</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001214764</rptOwnerCik>
            <rptOwnerName>NATINCO S A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>2, AVENUE CHARLES DE GAULLE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>L-1653 LUXEMBOURG</rptOwnerCity>
            <rptOwnerState>N4</rptOwnerState>
            <rptOwnerZipCode></rptOwnerZipCode>
            <rptOwnerStateDescription>LUXEMBOURG</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001468248</rptOwnerCik>
            <rptOwnerName>CAPITA TRUSTEES LTD AS TRUSTEES OF THE NATINEC SETTLEMENT</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>PO BOX 532</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>ST HELIER, JERSEY CHANNEL ISL</rptOwnerCity>
            <rptOwnerState></rptOwnerState>
            <rptOwnerZipCode>JE4 5UW</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2010-03-02</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>4275651</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>17.26</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4275650</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F2"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2010-03-02</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>4275650</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>17.26</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F2"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">The Shares were sold in a private transaction in exchange for non-cash consideration having this value.</footnote>
        <footnote id="F2">These securities were owned (i) indirectly by Natinco, (ii) indirectly by Capita Trustees Limited as Trustees of The Natinec Settlement (the &quot;Trust&quot;) and (iii) indirectly by the Trust.  The Trust owns all or substantially all of the equity interest in Natinco.</footnote>
    </footnotes>

    <remarks>Stirling Trustees Limited (formerly trustee of the Natinec Trust under The Natinec Settlement Trust dated January 9, 2003) was merged into Capita Trustees Limited (currently trustee of The Natinec Settlement).</remarks>

    <ownerSignature>
        <signatureName>Pierre Lentz, as a Director of Natinco, S.A.</signatureName>
        <signatureDate>2010-03-02</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>John Seil, as a Director of Natinco, S.A.</signatureName>
        <signatureDate>2010-03-02</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>James Muir, as a Director of Capita Trustees Limited as Trustees of The Natinec Settlement</signatureName>
        <signatureDate>2010-03-02</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Roger Colyer, as a Director of Capita Trustees Limited as Trustees of The Natinec Settlement</signatureName>
        <signatureDate>2010-03-02</signatureDate>
    </ownerSignature>
</ownershipDocument>
