  All existing data on neutrino oscillations (including those from the LSND
experiment) imply a four neutrino scheme with six different allowed mass
patterns. Some of the latter are shown to be disfavored by using a conservative
upper bound on the $\beta beta 0 \nu$ nuclear decay rate, if neutrinos are
assumed to be Majorana particles. Comparisons are also made with restrictions
from tritium $\beta$-decay and cosmology.
