Two hundred fifty-eight patients with clinically and serologically proven sporadic acute viral hepatitis during a period of past 7 years (January 1976-December 1982) were analyzed regarding epidemiology and outcome. The frequency of non-A, non-B (NANB) hepatitis was the highest among the three categories of viral hepatitis; 118 patients had hepatitis NANB (46%), 70 hepatitis A (27%), and 70 hepatitis B (27%). In NANB hepatitis, the mean age was older than in other categories of hepatitis and both sexes were equally affected, in contrast to the male predominance in types A and B. Chronic liver disease developed in 32% of patients with NANB hepatitis, but in none of patients with hepatitis type A or B. These results suggest that in Japan the infectious sources of hepatitis NANB virus(es) are more prevalent than those of hepatitis A and B viruses, and also suggest that one of the possible important factors for the high tendency to chronicity may be concerned with intimate contact with, or evolution from, asymptomatic NANB virus carriers.