  The CP violation required in leptogenesis may have different origin, but in
an effective theory they all are related to the rephasing invariant CP
violating measure in the mixing matrix of the leptonic sector. We point out
that the maximum amount of CP violation in some models can be estimated with
our present knowledge of the neutrino mixing angles, which can help us
understand the CP violation in the generation of the lepton asymmetry of the
universe. For example, the possibility of leptogenesis may be ruled out in some
models from an knowledge of the effective neutrino mass matrix.
