  We present results from the KamLAND-Zen double-beta decay experiment based on
an exposure of 77.6 days with 129 kg of $^{136}$Xe. The measured two-neutrino
double-beta decay half-life of $^{136}$Xe is $T_{1/2}^{2\nu} = 2.38 \pm
0.02(stat) \pm 0.14(syst) \times 10^{21}$ yr, consistent with a recent
measurement by EXO-200. We also obtain a lower limit for the neutrinoless
double-beta decay half-life, $T_{1/2}^{0\nu} > 5.7 \times 10^{24}$ yr at 90%
confidence level (C.L.), which corresponds to almost a five-fold improvement
over previous limits.
