  We present a measurement of CP-violating asymmetries in B0 -> pi+pi- decays
based on a 41.8 fb-1 data sample collected at the Upsilon(4S) resonance with
the Belle detector at the KEKB asymmetric-energy e+e- collider. We fully
reconstruct one neutral B meson as a B0 -> pi+pi- CP eigenstate and identify
the flavor of the accompanying B meson from its decay products. From the
asymmetry in the distribution of the time intervals between the two B meson
decay points, we obtain the CP-violating asymmetry parameters Spipi = -1.21
+0.38/-0.27(stat) +0.16/-0.13(syst) and Apipi = +0.94 +0.25/-0.31(stat) +/-
0.09(syst).
