A prospective field study was carried out during an epidemic of non-A non-B hepatitis for determining the incidence and severity of hepatitis in pregnant women, nonpregnant women of child bearing age and men (15 to 45 years old). In 36 (17.3 percent) of 208 pregnant women viral hepatitis developed, as compared to 71 (2.1 percent) of 3,350 nonpregnant women and 107 (2.8 percent) of 3,822 men. The incidence of disease in pregnant women was higher than in the two control groups. The incidence of viral hepatitis in the first, second and third trimesters was 8.8 percent, 19.4 percent, and 18.6 percent, respectively. The incidence in all three trimesters was higher, when compared to that in nonpregnant women. In eight pregnant women (22.2 percent) with viral hepatitis, fulminant hepatic failure developed, as compared to its occurrence in three men (2.8 percent) and in no nonpregnant women. This significantly increased incidence of fulminant hepatitis in pregnancy was indicative of a greater severity of hepatitis during pregnancy. Increased susceptibility to fulminant hepatitis was observed exclusively in the last trimester. Nonfulminant viral hepatitis did not influence the course of pregnancy or fetal well-being. Fetal loss in fatal fulminant hepatitis was a consequence of maternal death and could not be ascribed to direct effect on the fetus or pregnancy.