A waterborne epidemic of non-A, non-B hepatitis occurred in Medea, an Algerian town of 68,000 inhabitants during the autumn and winter of 1980-1981. About 6 weeks before the epidemic, there had been a chlorination failure and an accidental sewage contamination of the river water that supplied large parts of the town. From October, 1980, to January, 1981, 788 cases were notified. This epidemic affected mostly young adults, who usually recovered; however, a high rate of mortality was noted among pregnant women (nine patients, nine deaths). Serological examinations performed on 57 hospitalized patients excluded both hepatitis A and B. The waterborne nature of the epidemic was suggested by a study of the case distribution according to water supplies. The epidemic was severe in the areas supplied by the untreated river water. Few cases were reported in the areas receiving spring water, where chlorination never failed. A mild incidence of infection was noted in the areas supplied by a mixture of untreated river water and chlorinated spring water.