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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2009-01-01</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>2009-01-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>2059</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>50006.519</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>33.57</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2009-01-01</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>2009-01-01</value>
                <footnoteId id="F5"/>
            </exerciseDate>
            <expirationDate>
                <value>2019-01-01</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>5000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>30000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Restricted Stock granted by Issuer pursuant to the Ventas, Inc. 2006 Stock Plan for Directors (the &quot;Stock Plan&quot;).  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 2,131.952 shares acquired between March 30, 2008 and December 30, 2008 under the Ventas, Inc. Distribution Reinvestment and Stock Purchase Plan.</footnote>
        <footnote id="F3">Includes 1,236.497 shares acquired between March 4, 2008 and January 4, 2009 under the Ventas Employee and Director Stock Purchase Plan.</footnote>
        <footnote id="F4">Stock Options granted by Issuer pursuant to the Stock Plan.</footnote>
        <footnote id="F5">These options vest in two equal annual installments beginning on the date of grant.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Sheli Z. Rosenberg, By: T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2009-01-05</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0303
4
2009-01-01
0000740260
VENTAS INC
VTR
0001198222
ROSENBERG SHELI  Z
TWO NORTH RIVERSIDE PLAZA, SUITE 600
CHICAGO
IL
60606
1
0
0
0
Common Stock
2009-01-01
4
A
0
2059
0
A
50006.519
D
Stock Option (Right to Buy)
33.57
2009-01-01
4
A
0
5000
0
A
2009-01-01
2019-01-01
Common Stock
5000
30000
D
Restricted Stock granted by Issuer pursuant to the Ventas, Inc. 2006 Stock Plan for Directors (the "Stock Plan").  This Restricted Stock vests in two equal annual installments beginning on the first anniversary of the date of the grant.
Includes 2,131.952 shares acquired between March 30, 2008 and December 30, 2008 under the Ventas, Inc. Distribution Reinvestment and Stock Purchase Plan.
Includes 1,236.497 shares acquired between March 4, 2008 and January 4, 2009 under the Ventas Employee and Director Stock Purchase Plan.
Stock Options granted by Issuer pursuant to the Stock Plan.
These options vest in two equal annual installments beginning on the date of grant.
Sheli Z. Rosenberg, By: T. Richard Riney, Attorney-In-Fact
2009-01-05