  We consider the scheme with mixing of three neutrinos and a mass hierarchy.
We shown that, under the natural assumptions that massive neutrinos are
Majorana particles and there are no unlikely fine-tuned cancellations among the
contributions of the different neutrino masses, the results of solar neutrino
experiments imply a lower bound for the effective Majorana mass in neutrinoless
double-beta decay. We also discuss briefly neutrinoless double-beta decay in
schemes with mixing of four neutrinos. We show that one of them is favored by
the data.
