  The production of W bosons in association with two jets in proton-proton
collisions at a centre-of-mass energy of sqrt(s)=7 TeV has been analysed for
the presence of double-parton interactions using data corresponding to an
integrated luminosity of 36/pb, collected with the ATLAS detector at the LHC.
The fraction of events arising from double-parton interactions, fDP(D) has been
measured through the momentum balance between the two jets and amounts to
fDP(D) = 0.08 +- 0.01 (stat.) +- 0.02 (sys) for jets with transverse momentum
PT > 20 GeV and rapidity |y|<2.8. This corresponds to a measurement of the
effective area parameter for hard double-parton interactions of \sigma_eff = 15
+- 3 (stat.) +5 -3 (sys.) mb.
