  If neutrino masses and mixings are suitable to explain the atmospheric and
solar neutrino fluxes, this amounts to contributions to FCNC processes, in
particular mu --> e, gamma. If the theory is supersymmetric and the origin of
the masses is a see-saw mechanism, we show that the prediction for BR(mu --> e,
gamma) is in general larger than the experimental upper bound, especially if
the largest Yukawa coupling is O(1) and the solar data are explained by a large
angle MSW effect, which recent analyses suggest as the preferred scenario. Our
analysis is bottom-up and completely general, i.e. it is based just on
observable low-energy data. The work generalizes previous results of the
literature, identifying the dominant contributions. Application of the results
to scenarios with approximate top-neutrino unification, like SO(10) models,
rules out most of them unless the leptonic Yukawa matrices satisfy very precise
requirements. Other possible ways-out, like gauge mediated SUSY breaking, are
also discussed.
