In studies of Lassa fever in Sierra Leone, the prevalence. of human antibody to Lassa virus ranged from 8% to 52%. Mastomys natalensis, the reservoir of Lassa virus, constituted 50%–60% of the rodents captured in houses but only 10%–20% of those captured in surrounding agriculture and bush areas (χ² = 90.2, P < 10−6 , dƒ = 1), a finding suggesting that houses are the most-important location for transmission of Lassa virus. Viral infection of Mastomys from houses ranged from 0% to 80%. The incidence of seroconversions in susceptible persons ranged from 5% to 22% per year; the ratio of illness to infection ranged from 9% to 26%, and the proportion of febrile illness associated with seroconversion was 5%–14%. Eightfold rises in titer of antibody occurred in 1%–18% of the antibody-positive population, a result suggesting reinfection. We estimate the ratio of fatalities to infection to be 1%–2%, a rate lower than estimates based on hospitalized cases. The high incidence of Lassa fever makes it a major problem in West Africa.