  Oscillations between ordinary and sterile neutrinos can generate large
neutrino asymmetries in the early universe. These asymmetries can significantly
affect big bang nucleosynthesis (BBN) through modification of nuclear reaction
rates. We study this phenomenon within a model consisting of the three ordinary
neutrinos plus one sterile neutrino that can be motivated by the neutrino
anomalies and the dark matter problem. We calculate how the lepton asymmetries
produced evolve at temperatures where they impact on BBN. The effect of the
asymmetries on primordial helium production is determined, leading to an
effective number of neutrino flavours during BBN of either about 2.7 or 3.1
depending on the sign of the lepton asymmetry.
