<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>5</documentType>
<periodOfReport>2004-01-07</periodOfReport>
<notSubjectToSection16>0</notSubjectToSection16>
    <form3HoldingsReported>0</form3HoldingsReported>

    <form4TransactionsReported>0</form4TransactionsReported>

    <issuer>
        <issuerCik>0000703559</issuerCik>
        <issuerName>HUDSON UNITED BANCORP</issuerName>
        <issuerTradingSymbol>HU</issuerTradingSymbol>
</issuer>
<reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001185286</rptOwnerCik>
            <rptOwnerName>THOMAS NELSON</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1></rptOwnerStreet1>
            <rptOwnerStreet2>1000 MACARTHUR BOULEVARD</rptOwnerStreet2>
            <rptOwnerCity>MAHWAH</rptOwnerCity>
            <rptOwnerState>NJ</rptOwnerState>
            <rptOwnerZipCode>07430</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>0</isDirector>
<isOfficer>1</isOfficer>
<isTenPercentOwner>0</isTenPercentOwner>
<isOther>0</isOther>
<officerTitle>Executive Vice President</officerTitle>
<otherText></otherText>
</reportingOwnerRelationship>
</reportingOwner>
<nonDerivativeTable>
<nonDerivativeTransaction>
<securityTitle>
<value>Hudson United Bancorp Common Stock</value>
</securityTitle>
<transactionDate>
<value>2004-01-07</value>
</transactionDate>
<deemedExecutionDate>
</deemedExecutionDate>
<transactionCoding>
<transactionFormType>5</transactionFormType>
<transactionCode>J</transactionCode>
<equitySwapInvolved>0</equitySwapInvolved>
</transactionCoding>
<transactionTimeliness>
<value></value>
</transactionTimeliness>
<transactionAmounts>
<transactionShares>
<value>0</value>
</transactionShares>
<transactionPricePerShare>
<value>0</value>
</transactionPricePerShare>
<transactionAcquiredDisposedCode>
<value>A</value>
</transactionAcquiredDisposedCode>
</transactionAmounts>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
<value>10443.1398</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature>
<directOrIndirectOwnership>
<value>I</value>
</directOrIndirectOwnership>
<natureOfOwnership>
<value>401K</value>
</natureOfOwnership>
</ownershipNature>
</nonDerivativeTransaction>
<nonDerivativeTransaction>
<securityTitle>
<value>Hudson United Bancorp Common Stock</value>
</securityTitle>
<transactionDate>
<value>2004-01-07</value>
</transactionDate>
<deemedExecutionDate>
</deemedExecutionDate>
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<transactionTimeliness>
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<transactionPricePerShare>
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<transactionAcquiredDisposedCode>
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<ownershipNature>
<directOrIndirectOwnership>
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<natureOfOwnership>
<value></value>
</natureOfOwnership>
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</securityTitle>
<transactionDate>
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<deemedExecutionDate>
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<transactionTimeliness>
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<transactionAcquiredDisposedCode>
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<sharesOwnedFollowingTransaction>
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</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature>
<directOrIndirectOwnership>
<value>D</value>
</directOrIndirectOwnership>
<natureOfOwnership>
<value>Restricted</value>
</natureOfOwnership>
</ownershipNature>
</nonDerivativeTransaction>
</nonDerivativeTable>
<derivativeTable>
<derivativeTransaction>
<securityTitle>
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</securityTitle>
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</conversionOrExercisePrice>
<transactionDate>
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</transactionDate>
<deemedExecutionDate>
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<transactionTimeliness>
<value></value>
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<transactionAmounts>
<transactionShares>
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<transactionPricePerShare>
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</transactionPricePerShare>
<transactionAcquiredDisposedCode>
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</transactionAmounts>
<exerciseDate>
<value>1995-06-13</value>
</exerciseDate>
<expirationDate>
<value>2004-12-12</value>
</expirationDate>
<underlyingSecurity>
<underlyingSecurityTitle>
<value>Stock Options</value>
</underlyingSecurityTitle>
<underlyingSecurityShares>
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</underlyingSecurityShares>
</underlyingSecurity>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
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</postTransactionAmounts>
<ownershipNature>
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<natureOfOwnership>
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</natureOfOwnership>
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<derivativeTransaction>
<securityTitle>
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<transactionDate>
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<deemedExecutionDate>
</deemedExecutionDate>
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<transactionTimeliness>
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<transactionAmounts>
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<transactionPricePerShare>
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<transactionAcquiredDisposedCode>
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<expirationDate>
<value>2004-12-12</value>
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<underlyingSecurity>
<underlyingSecurityTitle>
<value>Stock Options</value>
</underlyingSecurityTitle>
<underlyingSecurityShares>
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</underlyingSecurity>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
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</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature>
<directOrIndirectOwnership>
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<natureOfOwnership>
<value></value>
</natureOfOwnership>
</ownershipNature>
</derivativeTransaction>
<derivativeTransaction>
<securityTitle>
<value>Hudson United Bancorp Common Stock</value>
</securityTitle>
<conversionOrExercisePrice>
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</conversionOrExercisePrice>
<transactionDate>
<value>2004-01-07</value>
</transactionDate>
<deemedExecutionDate>
</deemedExecutionDate>
<transactionCoding>
<transactionFormType>5</transactionFormType>
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<transactionTimeliness>
<value></value>
</transactionTimeliness>
<transactionAmounts>
<transactionShares>
<value>0</value>
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<transactionPricePerShare>
<value>10.362</value>
</transactionPricePerShare>
<transactionAcquiredDisposedCode>
<value>A</value>
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</transactionAmounts>
<exerciseDate>
<value>1997-06-13</value>
</exerciseDate>
<expirationDate>
<value>2004-12-12</value>
</expirationDate>
<underlyingSecurity>
<underlyingSecurityTitle>
<value>Stock Options</value>
</underlyingSecurityTitle>
<underlyingSecurityShares>
<value>0</value>
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</underlyingSecurity>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
<value>5571</value>
</sharesOwnedFollowingTransaction>
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<ownershipNature>
<directOrIndirectOwnership>
<value>D</value>
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<natureOfOwnership>
<value></value>
</natureOfOwnership>
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</derivativeTransaction>
<derivativeTransaction>
<securityTitle>
<value>Hudson United Bancorp Common Stock</value>
</securityTitle>
<conversionOrExercisePrice>
<value>0</value>
</conversionOrExercisePrice>
<transactionDate>
<value>2004-01-07</value>
</transactionDate>
<deemedExecutionDate>
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<transactionCoding>
<transactionFormType>5</transactionFormType>
<transactionCode>J</transactionCode>
<equitySwapInvolved>0</equitySwapInvolved>
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<transactionTimeliness>
<value></value>
</transactionTimeliness>
<transactionAmounts>
<transactionShares>
<value>0</value>
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<transactionPricePerShare>
<value>10.362</value>
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<transactionAcquiredDisposedCode>
<value>A</value>
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<exerciseDate>
<value>1998-06-13</value>
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<value>2004-12-12</value>
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<underlyingSecurity>
<underlyingSecurityTitle>
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<sharesOwnedFollowingTransaction>
<value>5571</value>
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<directOrIndirectOwnership>
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<value></value>
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</derivativeTransaction>
<derivativeTransaction>
<securityTitle>
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<value></value>
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<transactionShares>
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<value>10.362</value>
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<transactionAcquiredDisposedCode>
<value>A</value>
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<exerciseDate>
<value>1999-06-13</value>
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<expirationDate>
<value>2004-12-12</value>
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<underlyingSecurity>
<underlyingSecurityTitle>
<value>Stock Options</value>
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<sharesOwnedFollowingTransaction>
<value>5571</value>
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<value>D</value>
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<value></value>
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</derivativeTransaction>
<derivativeTransaction>
<securityTitle>
<value>Hudson United Bancorp Common Stock</value>
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<value>0</value>
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<transactionDate>
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<transactionTimeliness>
<value></value>
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<value>0</value>
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<value>17.367</value>
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<value>A</value>
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<value>2001-11-01</value>
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<value>2006-11-01</value>
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<underlyingSecurity>
<underlyingSecurityTitle>
<value>Stock Options</value>
</underlyingSecurityTitle>
<underlyingSecurityShares>
<value>0</value>
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<sharesOwnedFollowingTransaction>
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<directOrIndirectOwnership>
<value>D</value>
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<natureOfOwnership>
<value></value>
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<derivativeTransaction>
<securityTitle>
<value>Hudson United Bancorp Common Stock</value>
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<value>2004-01-07</value>
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<transactionTimeliness>
<value></value>
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<value>0</value>
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<transactionPricePerShare>
<value>28.972</value>
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<transactionAcquiredDisposedCode>
<value>A</value>
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<exerciseDate>
<value>2002-11-19</value>
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<expirationDate>
<value>2007-11-19</value>
</expirationDate>
<underlyingSecurity>
<underlyingSecurityTitle>
<value>Stock Options</value>
</underlyingSecurityTitle>
<underlyingSecurityShares>
<value>0</value>
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<sharesOwnedFollowingTransaction>
<value>14587</value>
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<ownershipNature>
<directOrIndirectOwnership>
<value>D</value>
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<natureOfOwnership>
<value></value>
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</derivativeTransaction>
<derivativeTransaction>
<securityTitle>
<value>Hudson United Bancorp Common Stock</value>
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<value>0</value>
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<transactionDate>
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</deemedExecutionDate>
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<transactionFormType>5</transactionFormType>
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<value></value>
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<transactionShares>
<value>0</value>
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<transactionPricePerShare>
<value>29.127</value>
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<transactionAcquiredDisposedCode>
<value>A</value>
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<exerciseDate>
<footnoteId id="F1"/>
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<expirationDate>
<value>2009-03-22</value>
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<underlyingSecurity>
<underlyingSecurityTitle>
<value>Stock Options</value>
</underlyingSecurityTitle>
<underlyingSecurityShares>
<value>0</value>
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<sharesOwnedFollowingTransaction>
<value>113</value>
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<ownershipNature>
<directOrIndirectOwnership>
<value>D</value>
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<natureOfOwnership>
<value></value>
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<derivativeTransaction>
<securityTitle>
<value>Hudson United Bancorp Common Stock</value>
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<value>0</value>
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<transactionDate>
<value>2004-01-07</value>
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<deemedExecutionDate>
</deemedExecutionDate>
<transactionCoding>
<transactionFormType>5</transactionFormType>
<transactionCode>J</transactionCode>
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<transactionTimeliness>
<value></value>
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<transactionAmounts>
<transactionShares>
<value>0</value>
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<transactionPricePerShare>
<value>27.46</value>
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<transactionAcquiredDisposedCode>
<value>A</value>
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<exerciseDate>
<footnoteId id="F1"/>
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<expirationDate>
<value>2011-12-06</value>
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<underlyingSecurity>
<underlyingSecurityTitle>
<value>Stock Options</value>
</underlyingSecurityTitle>
<underlyingSecurityShares>
<value>0</value>
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</underlyingSecurity>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
<value>10000</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature>
<directOrIndirectOwnership>
<value>D</value>
</directOrIndirectOwnership>
<natureOfOwnership>
<value></value>
</natureOfOwnership>
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</derivativeTransaction>
<derivativeTransaction>
<securityTitle>
<value>Hudson United Bancorp Common Stock</value>
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<conversionOrExercisePrice>
<value>0</value>
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<transactionDate>
<value>2004-01-07</value>
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<deemedExecutionDate>
</deemedExecutionDate>
<transactionCoding>
<transactionFormType>5</transactionFormType>
<transactionCode>J</transactionCode>
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<transactionTimeliness>
<value></value>
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<transactionShares>
<value>0</value>
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<transactionPricePerShare>
<value>27.39</value>
</transactionPricePerShare>
<transactionAcquiredDisposedCode>
<value>A</value>
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<exerciseDate>
<footnoteId id="F1"/>
</exerciseDate>
<expirationDate>
<value>2012-08-07</value>
</expirationDate>
<underlyingSecurity>
<underlyingSecurityTitle>
<value>Stock Options</value>
</underlyingSecurityTitle>
<underlyingSecurityShares>
<value>0</value>
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</underlyingSecurity>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
<value>14000</value>
</sharesOwnedFollowingTransaction>
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<ownershipNature>
<directOrIndirectOwnership>
<value>D</value>
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<natureOfOwnership>
<value></value>
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</derivativeTransaction>
</derivativeTable>
<footnotes>
<footnote id="F1">1/3 after 3rd year; 1/3 after 4th year; and 1/3 after 5th year.</footnote>
</footnotes>
<remarks></remarks>
<ownerSignature>
<signatureName>Thomas Nelson</signatureName>
<signatureDate>2004-01-07</signatureDate>
</ownerSignature>
</ownershipDocument>

