  A precise determination of |V_{ub}| can be obtained exploiting the sum rule
for inclusive charmless semileptonic B-meson decays. The sum rule is derived on
the basis of light-cone expansion and b-flavored quantum number conservation.
The sum rule does not receive any perturbative QCD correction. In this
determination of |V_{ub}|, there is no perturbative QCD uncertainty, while the
dominant hadronic uncertainty is avoided. Moreover, this method is not only
theoretically quite clean, but experimentally also very efficient in the
discrimination between B\to X_u\ell\nu signal and B\to X_c\ell\nu background.
The sum rule requires measuring the lepton pair spectrum. We analyze the lepton
pair spectrum, including the leading perturbative and nonperturbative QCD
corrections.
