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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
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 qsa9SCMB6kuuwPDXSfrG2w==

<SEC-DOCUMENT>0001104659-10-017668.txt : 20100331
<SEC-HEADER>0001104659-10-017668.hdr.sgml : 20100331
<ACCEPTANCE-DATETIME>20100331150546
ACCESSION NUMBER:		0001104659-10-017668
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20100329
FILED AS OF DATE:		20100331
DATE AS OF CHANGE:		20100331

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			ARCA biopharma, Inc.
		CENTRAL INDEX KEY:			0000907654
		STANDARD INDUSTRIAL CLASSIFICATION:	IN VITRO & IN VIVO DIAGNOSTIC SUBSTANCES [2835]
		IRS NUMBER:				363855489
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		8001 ARISTA PLACE
		STREET 2:		SUITE 200
		CITY:			BROOMFIELD
		STATE:			CO
		ZIP:			80021
		BUSINESS PHONE:		720-940-2200

	MAIL ADDRESS:	
		STREET 1:		8001 ARISTA PLACE
		STREET 2:		SUITE 200
		CITY:			BROOMFIELD
		STATE:			CO
		ZIP:			80021

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	NUVELO INC
		DATE OF NAME CHANGE:	20030203

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	HYSEQ INC
		DATE OF NAME CHANGE:	19970610

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Atlas Venture Fund VII L P
		CENTRAL INDEX KEY:			0001346450
		STATE OF INCORPORATION:			DE

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-22873
		FILM NUMBER:		10718536

	BUSINESS ADDRESS:	
		STREET 1:		890 WINTER STREET
		CITY:			WALTHAM
		STATE:			MA
		ZIP:			02451
		BUSINESS PHONE:		7816221700

	MAIL ADDRESS:	
		STREET 1:		890 WINTER STREET
		CITY:			WALTHAM
		STATE:			MA
		ZIP:			02451
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>a4.xml
<DESCRIPTION>4
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-03-29</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000907654</issuerCik>
        <issuerName>ARCA biopharma, Inc.</issuerName>
        <issuerTradingSymbol>ABIO</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001346450</rptOwnerCik>
            <rptOwnerName>Atlas Venture Fund VII L P</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ATLAS VENTUR</rptOwnerStreet1>
            <rptOwnerStreet2>890 WINTER STREET, SUITE 320</rptOwnerStreet2>
            <rptOwnerCity>WALTHAM</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02451</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2010-03-29</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>140000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>7.56</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1337612</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Represents 1,262,163 shares of Common Stock and 75,449 warrants to purchase shares of Common Stock held by Atlas Venture Fund VII, L.P. These shares are held directly by AV VII.  Atlas Venture Associates VII, L.P., or AVA VII LP, is the sole general partner of AV VII.  Each of Atlas VII and AVA VII LP disclaims beneficial ownership of such shares except to the extent of it's pecuniary interest therein.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Atlas Venture Fund VII, L.P. By: Atlas Venture Associates VII, L.P. Its General Partner Atlas Venture Fund VII, L.P. By: Atlas Venture Associates VII, L.P. Its General Partner /s/ Kristen Laguerre, Vice President</signatureName>
        <signatureDate>2010-03-31</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Atlas Venture Associates VII, L.P. By: Atlas Venture Associates VII, Inc. Its General Partner /s/ Kristen Laguerre, Vice President</signatureName>
        <signatureDate>2010-03-31</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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