  We consider some consequences of the presence of a cosmological lepton
asymmetry in the form of neutrinos. Relic neutrino degeneracy enhances the
contribution of massive neutrinos to the present energy density of the
Universe, and modifies the power spectrum of radiation and matter. Comparing
with current observations of cosmic microwave background anisotropies and large
scale structure, we derive some constraints on the neutrino degeneracy and on
the spectral index in the case of a flat Universe with a cosmological constant.
