  We study leptogenesis from the out-of-equilibrium decays of the lightest
heavy neutrino $N_1$ in the medium (low) temperature regime, $T\lsim 10^{12}$
GeV ($10^{10}$ GeV), where the rates of processes mediated by the $\tau$ (and
$\mu$) Yukawa coupling are non negligible, implying that the effects of lepton
flavors must be taken into account. We find important quantitative and
qualitative differences with respect to the case where flavor effects are
ignored: (i) The cosmic baryon asymmetry can be enhanced by up to one order of
magnitude; (ii) The sign of the asymmetry can be opposite to what one would
predict from the sign of the total lepton asymmetry $\epsilon_1$; (iii)
Successful leptogenesis is possible even with $\epsilon_1=0$.
