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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-12-21</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>2012-12-21</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>3700186</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnotes (1), (2) and (3)</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5822.791</value>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Prometheus Senior Quarters LLC, a Delaware limited liability company (&quot;Prometheus&quot;), Lazard Senior Housing Partners LP, a Delaware limited partnership (&quot;LSHP&quot;), and LSHP Coinvestment Partnership I LP, a Delaware limited partnership (&quot;LSHP Coinvest&quot;), collectively, may be deemed to beneficially own 3,700,186 shares of the Issuer's common stock (the &quot;Shares&quot;).  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 (the &quot;Managing Members&quot; and, together with each of LSHP and LSHP Coinvest, the &quot;Funds&quot;).  Prior to the Transaction described herein, the general partner of LSHP was Lazard Senior Housing Partners GP LLC, a Delaware limited liability company (&quot;LSHP GP&quot;), the general partner of LSHP Coinvest was LSHP Coinvestment I GP LLC, a Delaware limited liability company (&quot;LSHP Coinvest GP&quot;), the general partner of each of the Managing Members</footnote>
        <footnote id="F2">[Continued from previous] was Lazard Freres Real Estate Investors L.L.C., a New York limited liability company (&quot;LFREI&quot;), and the managing member of each of LFREI, LSHP GP and LSHP Coinvest GP was Lazard Alternative Investments LLC, a Delaware limited liability company (&quot;LAI&quot;).  Prior to the Transaction described herein, Mr. Lustig, 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, may have been deemed to be a beneficial owner of the Shares to the extent of any pecuniary interest he may have had in the Shares.</footnote>
        <footnote id="F3">[Continued from previous] On December 21, 2012, the Issuer acquired 100% of the Funds (the &quot;Transaction&quot;), including the Shares and all other assets owned by the Funds, for an aggregate purchase price of approximately $242 million, plus the transferors' proportionate share of an additional $44 million payment made by the Issuer to extinguish its obligation related to the earnout arising from the 2011 merger transaction involving the Issuer pursuant to which the Shares were acquired.  Following the Transaction, Mr. Lustig does not have beneficial ownership of any of the Shares.</footnote>
        <footnote id="F4">As of December 21, 2012, Reporting Person also owns options to purchase 8,191 shares of Issuer's common stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Matthew J. Lustig, By: T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2012-12-26</signatureDate>
    </ownerSignature>
</ownershipDocument>
[{"type": "4", "sequence": "1", "filename": "edgar.xml", "description": "PRIMARY DOCUMENT", "secsgml_size_bytes": 6391}]
X0306
4
2012-12-21
0000740260
VENTAS INC
VTR
0001203367
LUSTIG MATTHEW J
30 ROCKEFELLER PLAZA
NEW YORK,
NY
10020
1
0
0
0
Common Stock
2012-12-21
4
S
0
3700186
D
0
I
See footnotes (1), (2) and (3)
Common Stock
5822.791
D
Prometheus Senior Quarters LLC, a Delaware limited liability company ("Prometheus"), Lazard Senior Housing Partners LP, a Delaware limited partnership ("LSHP"), and LSHP Coinvestment Partnership I LP, a Delaware limited partnership ("LSHP Coinvest"), collectively, may be deemed to beneficially own 3,700,186 shares of the Issuer's common stock (the "Shares").  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 (the "Managing Members" and, together with each of LSHP and LSHP Coinvest, the "Funds").  Prior to the Transaction described herein, the general partner of LSHP was Lazard Senior Housing Partners GP LLC, a Delaware limited liability company ("LSHP GP"), the general partner of LSHP Coinvest was LSHP Coinvestment I GP LLC, a Delaware limited liability company ("LSHP Coinvest GP"), the general partner of each of the Managing Members
[Continued from previous] was Lazard Freres Real Estate Investors L.L.C., a New York limited liability company ("LFREI"), and the managing member of each of LFREI, LSHP GP and LSHP Coinvest GP was Lazard Alternative Investments LLC, a Delaware limited liability company ("LAI").  Prior to the Transaction described herein, Mr. Lustig, 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, may have been deemed to be a beneficial owner of the Shares to the extent of any pecuniary interest he may have had in the Shares.
[Continued from previous] On December 21, 2012, the Issuer acquired 100% of the Funds (the "Transaction"), including the Shares and all other assets owned by the Funds, for an aggregate purchase price of approximately $242 million, plus the transferors' proportionate share of an additional $44 million payment made by the Issuer to extinguish its obligation related to the earnout arising from the 2011 merger transaction involving the Issuer pursuant to which the Shares were acquired.  Following the Transaction, Mr. Lustig does not have beneficial ownership of any of the Shares.
As of December 21, 2012, Reporting Person also owns options to purchase 8,191 shares of Issuer's common stock.
Matthew J. Lustig, By: T. Richard Riney, Attorney-In-Fact
2012-12-26