The Double Chooz experiment has observed 8249 candidate electron antineutrino events in 227.93 live days with 33.71 GW-ton-years (reactor power X detector mass X live time) exposure using a 10.3 m(3) fiducial volume detector located at 1050 m from the reactor cores of the Chooz nuclear power plant in France. The expectation in case of theta(13) = 0 is 8937 events. The deficit is interpreted as evidence of electron antineutrino disappearance. From a rate plus spectral shape analysis we find sin(2)2 theta(13) = 0.109 +/- 0.030(stat) +/- 0.025(syst). The data exclude the no-oscillation hypothesis at 99.8% CL (2.9 sigma).