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

    <schemaVersion>X0304</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-01-22</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001346588</rptOwnerCik>
            <rptOwnerName>Brehl Robert J</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>10350 ORMSBY PARK PLACE, SUITE 300</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>LOUISVILLE,</rptOwnerCity>
            <rptOwnerState>KY</rptOwnerState>
            <rptOwnerZipCode>40223</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chief Acct. Off. &amp; Controller</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-01-22</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>179</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>56.42</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>18002.878</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">Represents shares withheld to pay the taxes on the vesting of restricted stock granted on January 22, 2008 under the Ventas, Inc. 2006 Incentive Plan.</footnote>
        <footnote id="F2">Includes 385.918 shares acquired between March 31, 2011 and December 30, 2011 under the Ventas, Inc. Distribution Reinvestment and Stock Purchase Plan.</footnote>
        <footnote id="F3">Includes 349.939 shares acquired between March 1, 2011 and January 23, 2012 under the Ventas Employee and Director Stock Purchase Plan.</footnote>
        <footnote id="F4">As of January 22, 2012, Reporting Person also owns options to purchase 27,385 shares of Issuer's common stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Robert J. Brehl, By: T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2012-01-24</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0304
4
2012-01-22
0000740260
VENTAS INC
VTR
0001346588
Brehl Robert J
10350 ORMSBY PARK PLACE, SUITE 300
LOUISVILLE,
KY
40223
0
1
0
0
Chief Acct. Off. & Controller
Common Stock
2012-01-22
4
F
0
179
56.42
D
18002.878
D
Represents shares withheld to pay the taxes on the vesting of restricted stock granted on January 22, 2008 under the Ventas, Inc. 2006 Incentive Plan.
Includes 385.918 shares acquired between March 31, 2011 and December 30, 2011 under the Ventas, Inc. Distribution Reinvestment and Stock Purchase Plan.
Includes 349.939 shares acquired between March 1, 2011 and January 23, 2012 under the Ventas Employee and Director Stock Purchase Plan.
As of January 22, 2012, Reporting Person also owns options to purchase 27,385 shares of Issuer's common stock.
Robert J. Brehl, By: T. Richard Riney, Attorney-In-Fact
2012-01-24