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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-02-02</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 Acctg. Off.&amp; Controller</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2010-02-02</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>25</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>43.40</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>15624.038</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </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 February 2, 2007, under the Ventas, Inc. 2006 Incentive Plan.</footnote>
        <footnote id="F2">Includes 38.402 shares acquired between January 20, 2010 and February 3, 2010 under the Ventas Employee and Director Stock Purchase Plan.</footnote>
        <footnote id="F3">As of February 2, 2010, Reporting Person also owns options to purchase 20,700 shares of Issuer's common stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Robert J. Brehl, By: T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2010-02-04</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0303
4
2010-02-02
0000740260
VENTAS INC
VTR
0001346588
Brehl Robert J
10350 ORMSBY PARK PLACE, SUITE 300
LOUISVILLE
KY
40223
0
1
0
0
Chief Acctg. Off.& Controller
Common Stock
2010-02-02
4
F
0
25
43.40
D
15624.038
D
Represents shares withheld to pay the taxes on the vesting of restricted stock granted on February 2, 2007, under the Ventas, Inc. 2006 Incentive Plan.
Includes 38.402 shares acquired between January 20, 2010 and February 3, 2010 under the Ventas Employee and Director Stock Purchase Plan.
As of February 2, 2010, Reporting Person also owns options to purchase 20,700 shares of Issuer's common stock.
Robert J. Brehl, By: T. Richard Riney, Attorney-In-Fact
2010-02-04