  Inverting the type-I seesaw formula, we reconstruct the mass matrix of the
heavy right-handed neutrinos $N_i$. We analyze how the data on neutrino
oscillations affect the structure of this matrix. Under the assumption of
hierarchical Dirac-type neutrino masses $m_{Di}$, we compute the mixing angles
among $N_i$, their masses $M_i$ and the lepton asymmetries $\epsilon_i$
generated in their decays. Unless special cancellations take place, one finds
$M_i\propto m_{Di}^2$ and the generated baryon-to-photon ratio $\eta_B$ is much
smaller than the observed value. We show that successful baryogenesis via
leptogenesis occurs in a unique special case, which corresponds to mass
degeneracy and maximal mixing of $N_1$ and $N_2$.
