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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2013-01-01</periodOfReport>

    <issuer>
        <issuerCik>0000740260</issuerCik>
        <issuerName>VENTAS INC</issuerName>
        <issuerTradingSymbol>VTR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001198222</rptOwnerCik>
            <rptOwnerName>ROSENBERG SHELI  Z</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>155 NORTH WACKER DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>CHICAGO,</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60606</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2013-01-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1545</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>73730.631</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Restricted stock granted by Issuer pursuant to the Ventas, Inc. 2012 Incentive Plan. This restricted stock vests in two equal annual installments beginning on the first anniversary of the date of the grant.</footnote>
        <footnote id="F2">Includes 2,292.157 shares acquired between January 4, 2012 and January 2, 2013 under the Ventas Employee and Director Stock Purchase Plan.</footnote>
        <footnote id="F3">Includes 1,864.716 shares acquired between March 29, 2012 and December 28, 2012 under the Ventas, Inc. Distribution Reinvestment and Stock Purchase Plan.</footnote>
        <footnote id="F4">As of January 1, 2013, Reporting Person also owns options to purchase 45,000 shares of Issuer's common stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Sheli Z. Rosenberg, By: T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2013-01-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2013-01-01
0000740260
VENTAS INC
VTR
0001198222
ROSENBERG SHELI  Z
155 NORTH WACKER DRIVE
CHICAGO,
IL
60606
1
0
0
0
Common Stock
2013-01-01
4
A
0
1545
0
A
73730.631
D
Restricted stock granted by Issuer pursuant to the Ventas, Inc. 2012 Incentive Plan. This restricted stock vests in two equal annual installments beginning on the first anniversary of the date of the grant.
Includes 2,292.157 shares acquired between January 4, 2012 and January 2, 2013 under the Ventas Employee and Director Stock Purchase Plan.
Includes 1,864.716 shares acquired between March 29, 2012 and December 28, 2012 under the Ventas, Inc. Distribution Reinvestment and Stock Purchase Plan.
As of January 1, 2013, Reporting Person also owns options to purchase 45,000 shares of Issuer's common stock.
Sheli Z. Rosenberg, By: T. Richard Riney, Attorney-In-Fact
2013-01-03