  The lepton asymmetry that could be generated in the early universe through
oscillations of active to sterile neutrinos is calculated (almost) analytically
for small mixing angles, sin 2\theta < 10^{-2}. It is shown that for a mass
squared difference, \delta m^2=-1 eV^2 it may rise at most by 6 orders of
magnitude from the initial ``normal'' value of 10^{-10}, since the
back-reaction from the refraction index terminates this rise while the
asymmetry is still small. Only for very large mass differences, \delta m^2
about 10^9 eV^2, the lepton asymmetry could reach a significant magnitude
exceeding 0.1.
