The incidence of hepatitis in a general open population of Asian adults was estimated for the first time in this study. A group of 2445 students were first tested when they enrolled at National Taiwan University in 1977; approximately one third were susceptible to hepatitis A and another third to hepatitis B. Most of these students (92%) were retested shortly before their graduation in 1981 to determine the frequency of serologic conversions and clinical hepatitis which had occurred in the 3 1/2 years since they had entered the university. Among 704 susceptible to hepatitis A, 12 (1.7%) had undergone serologic conversions, 33% of which were associated with clinical illness diagnosed as hepatitis. Among 738 susceptible to hepatitis B, 39 (5.3%) had undergone serologic conversions, 12.8% of which were associated with clinical hepatitis. The annual incidence of new infections was 0.5% for hepatitis A and 1.5% for hepatitis B. An additional eight students among the 17 who had clinical hepatitis had no associated conversion of hepatitis A or hepatitis B markers, and were considered to have non-A, non-B hepatitis. No factors could be identified which were predictive of hepatitis risk. No difference in incidence was observed according to sex, type of residence, place of food consumption, or receipt of acupuncture or blood transfusion. Among the 39 students who experienced hepatitis B infections while at the university, there were 2.7% who became hepatitis B surface antigen (HBsAg) carriers. Thus the carrier frequency following hepatitis B infection among Chinese adults is the same or lower than that among Caucasian adults.