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<ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>4</documentType><periodOfReport>2003-11-25</periodOfReport>
    <issuer>
        <issuerCik>0001123313</issuerCik>
        <issuerName>KYPHON INC</issuerName>
        <issuerTradingSymbol>KYPH</issuerTradingSymbol></issuer><reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001237710</rptOwnerCik>
            <rptOwnerName>INVESTOR GROWTH CAPITAL LTD</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>NATIONAL WESTMINSTER HOUSE</rptOwnerStreet1>
            <rptOwnerStreet2>LE TRUCHOT ST. PETER PORT</rptOwnerStreet2>
            <rptOwnerCity>GUERNSEY CHANNEL ISLAND, UK</rptOwnerCity>
            <rptOwnerState></rptOwnerState>
            <rptOwnerZipCode>GYI 4PW</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
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Stock</value></securityTitle><transactionDate><value>2003-11-25</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>S</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>5250</value></transactionShares><transactionPricePerShare><value>26.12</value></transactionPricePerShare><transactionAcquiredDisposedCode><value>D</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>2809623</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></nonDerivativeTransaction><nonDerivativeTransaction><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>2003-11-25</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>S</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>5250</value></transactionShares><transactionPricePerShare><value>26.28</value></transactionPricePerShare><transactionAcquiredDisposedCode><value>D</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>2804373</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></nonDerivativeTransaction></nonDerivativeTable><footnotes/><remarks/><ownerSignature><signatureName>Henry Gooss for Investor Growth Capital Limited</signatureName><signatureDate>2003-11-26</signatureDate></ownerSignature><ownerSignature><signatureName>Michael Oporto for Investor Growth Capital Limited</signatureName><signatureDate>2003-11-26</signatureDate></ownerSignature></ownershipDocument>

