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, espe
cially if the largest Yakawa coupling is O(l) and the solar data are explai
ned by a large angle MSW effect, which recent analyses suggest as the prefe
rred scenario. Our analysis is bottom-up and completely general, i.e., it i
s based just on observable low-energy data. The work generalizes previous r
esults of the literature, identifying the dominant contributions. Applicati
on of the results to scenarios with approximate top-neutrino unification, l
ike SO(10) models, rules out most of them unless the leptonic Yukawa matric
es satisfy very precise requirements. Other possible ways-out, like gauge m
ediated SUSY breaking, are also discussed. (C) 2001 Elsevier Science B.V. A
ll rights reserved.