The NA50 experiment measures, at CERN SPS, the dimuon production in Pb-Pb collisions at 158 GeV/c incident momentum. In this paper an analysis of the intermediate mass region (1.5 less than or equal to M(mu mu)12.5 GeV/c(2)) is presented. Known dimuon sources in this interval of the spectrum are the e.m. annihilation of <q(q)over bar> pairs (i.e, Drell-Yan) and the semileptonic decay of charmed hadrons. The contribution of these processes is calculated by means of a fit to the measured invariant mass distribution (for Drell-Yan) and of a linear extrapolation of p-A experimental results obtained by NA38 (for charm). It will he shown that known sources do not account for all of the dimuon yield measured by NA50, i.e, an excess is observed. The size of the excess measured in Pb-Pb is larger than the one seen by NA38 in S-U collisions. The centrality dependence of this effect, both in S-U and Pb-Pb, is also discussed.