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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2011-05-13</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001203367</rptOwnerCik>
            <rptOwnerName>LUSTIG MATTHEW J</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>30 ROCKEFELLER PLAZA</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10020</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>2011-05-13</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>2656</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2656</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>24854060</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote (2) (3)</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>54.15</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2011-05-13</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>3191</value>
                    <footnoteId id="F4"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2011-05-13</value>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <value>2021-05-13</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>3191</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3191</value>
                    <footnoteId id="F5"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Restricted stock granted by the 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 grant.</footnote>
        <footnote id="F2">Prometheus Senior Quarters LLC (&quot;Prometheus&quot;), a Delaware limited liability company, Lazard Senior Housing Partners LP (&quot;LSHP&quot;), a Delaware limited partnership, and LSHP Coinvestment Partnership I LP (&quot;LSHP Coinvest&quot;), a Delaware limited partnership, collectively, may be deemed to beneficially own 24,854,060 shares of Issuer's common stock. The Managing Members of Prometheus are LFSRI II-CADIM Alternative Partnership L.P., LF Strategic Realty Investors II L.P. and LFSRI II Alternative Partnership L.P., each a Delaware limited partnership, and Lazard Fr?res Real Estate Investors L.L.C. (&quot;LFREI&quot;), a New York limited liability company, is the general partner of each such limited partnership. Lazard Senior Housing Partners GP LLC (&quot;LSHP GP&quot;), a Delaware limited liability company, is the general partner of LSHP. LSHP Coinvestment I GP LLC (&quot;LSHP Coinvest GP&quot;), a Delaware limited liability company, is the general partner of LSHP Coinvest.</footnote>
        <footnote id="F3">[Continued from previous] Lazard Alternative Investments LLC (&quot;LAI&quot;), a Delaware limited liability company, is the managing member of each of LFREI, LSHP GP and LSHP Coinvest GP. In his capacity as Managing Director of LAI, Managing Principal of LFREI and Managing Principal and Chief Executive Officer of each of LSHP GP and LSHP Coinvest GP, Mr. Lustig may be deemed to be the beneficial owner of the securities reported herein. Mr. Lustig expressly disclaims beneficial ownership of the securities reported herein, except to the extent of his pecuniary interest therein, if any.</footnote>
        <footnote id="F4">Options to purchase 3,191 shares of Issuer's common stock having an exercise price of $54.15 that were granted by the Issuer pursuant to the Stock Plan and vest one-half on the date of grant and one-half on the first anniversary of the date of grant.</footnote>
        <footnote id="F5">Represents total number of unexercised options to purchase shares of Issuer's common stock held by Mr. Lustig as of May 16, 2011.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Matthew J. Lustig, By: T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2011-05-16</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0303
4
2011-05-13
0000740260
VENTAS INC
VTR
0001203367
LUSTIG MATTHEW J
30 ROCKEFELLER PLAZA
NEW YORK
NY
10020
1
0
0
0
Common Stock
2011-05-13
4
A
0
2656
0
A
2656
D
Common Stock
24854060
I
See Footnote (2) (3)
Stock Option (Right to Buy)
54.15
2011-05-13
4
A
0
3191
0
A
2011-05-13
2021-05-13
Common Stock
3191
3191
D
Restricted stock granted by the 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 grant.
Prometheus Senior Quarters LLC ("Prometheus"), a Delaware limited liability company, Lazard Senior Housing Partners LP ("LSHP"), a Delaware limited partnership, and LSHP Coinvestment Partnership I LP ("LSHP Coinvest"), a Delaware limited partnership, collectively, may be deemed to beneficially own 24,854,060 shares of Issuer's common stock. The Managing Members of Prometheus are LFSRI II-CADIM Alternative Partnership L.P., LF Strategic Realty Investors II L.P. and LFSRI II Alternative Partnership L.P., each a Delaware limited partnership, and Lazard Fr?res Real Estate Investors L.L.C. ("LFREI"), a New York limited liability company, is the general partner of each such limited partnership. Lazard Senior Housing Partners GP LLC ("LSHP GP"), a Delaware limited liability company, is the general partner of LSHP. LSHP Coinvestment I GP LLC ("LSHP Coinvest GP"), a Delaware limited liability company, is the general partner of LSHP Coinvest.
[Continued from previous] Lazard Alternative Investments LLC ("LAI"), a Delaware limited liability company, is the managing member of each of LFREI, LSHP GP and LSHP Coinvest GP. In his capacity as Managing Director of LAI, Managing Principal of LFREI and Managing Principal and Chief Executive Officer of each of LSHP GP and LSHP Coinvest GP, Mr. Lustig may be deemed to be the beneficial owner of the securities reported herein. Mr. Lustig expressly disclaims beneficial ownership of the securities reported herein, except to the extent of his pecuniary interest therein, if any.
Options to purchase 3,191 shares of Issuer's common stock having an exercise price of $54.15 that were granted by the Issuer pursuant to the Stock Plan and vest one-half on the date of grant and one-half on the first anniversary of the date of grant.
Represents total number of unexercised options to purchase shares of Issuer's common stock held by Mr. Lustig as of May 16, 2011.
Matthew J. Lustig, By: T. Richard Riney, Attorney-In-Fact
2011-05-16