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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2008-01-01</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>TWO NORTH RIVERSIDE PLAZA, SUITE 600</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>2008-01-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1337</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>24080.092</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>45.25</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2008-01-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>5000</value>
                    <footnoteId id="F4"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2008-01-01</value>
                <footnoteId id="F5"/>
            </exerciseDate>
            <expirationDate>
                <value>2018-01-01</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>5000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>45000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Restricted Stock granted pursuant to the 2006 Stock Plan for Directors (the &quot;Stock Plan&quot;).  This Restricted Stock vests in two equal annual installments beginning January 1, 2009.</footnote>
        <footnote id="F2">Includes 44.020 Units acquired on December 28, 2007, under the Non-employee Director Deferred Stock Compensation Plan as a result of dividend equivalents credited with respect to dividend paid on Issuer's Common Stock on December 28, 2007.</footnote>
        <footnote id="F3">Includes 200.029 shares acquired on December 28, 2007 under the Ventas, Inc. Distribution Reinvestment and Stock Purchase Plan.</footnote>
        <footnote id="F4">Stock Options granted pursuant to the Stock Plan.</footnote>
        <footnote id="F5">These options vest in two equal annual installments beginning January 1, 2008.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Sheli Z. Rosenberg, By: T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2008-01-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0202
4
2008-01-01
0000740260
VENTAS INC
VTR
0001198222
ROSENBERG SHELI  Z
TWO NORTH RIVERSIDE PLAZA, SUITE 600
CHICAGO
IL
60606
1
0
0
0
Common Stock
2008-01-01
4
A
0
1337
0
A
24080.092
D
Stock Option (Right to Buy)
45.25
2008-01-01
4
A
0
5000
0
A
2008-01-01
2018-01-01
Common Stock
5000
45000
D
Restricted Stock granted pursuant to the 2006 Stock Plan for Directors (the "Stock Plan").  This Restricted Stock vests in two equal annual installments beginning January 1, 2009.
Includes 44.020 Units acquired on December 28, 2007, under the Non-employee Director Deferred Stock Compensation Plan as a result of dividend equivalents credited with respect to dividend paid on Issuer's Common Stock on December 28, 2007.
Includes 200.029 shares acquired on December 28, 2007 under the Ventas, Inc. Distribution Reinvestment and Stock Purchase Plan.
Stock Options granted pursuant to the Stock Plan.
These options vest in two equal annual installments beginning January 1, 2008.
Sheli Z. Rosenberg, By: T. Richard Riney, Attorney-In-Fact
2008-01-03