  We solve the problem of coherent Mikheyev-Smirnov-Wolfenstein (MSW) resonant
active-to-sterile neutrino flavor conversion driven by an initial lepton number
in the early universe. We find incomplete destruction of lepton number in this
process and a sterile neutrino energy distribution with a distinctive cusp and
high energy tail. These features imply alteration of the non-zero lepton number
primordial nucleosynthesis paradigm when there exist sterile neutrinos with
rest masses ~ 1 eV. This could result in better light element probes of
(constraints on) these particles.
