In order to investigate the factors that make viral hepatitis type A serious, clinical pictures were analyzed in 269 patients with viral hepatitis type A broken out during the past 8 years. In 1986 incidence of patients with severe hepatic dysfunction was higher and the average age of them was as high as 36.8 +/- 12.8 years old. The percentage of patients over 40 years of age was increasing from 1979 to 1986. Moreover in 6 patients with fulminant hepatitis type A, 4 of them (66.7%) were older than 50 years. Analysis of the clinical courses of 9 patients associated with underlying chronic liver disease disclosed serious hepatic dysfunctions in 7 out of 9. And the prolonged course of abnormal transaminase levels was observed in one of them. Therefore, aging and underlying chronic liver disease were considered as main factors that make viral hepatitis type A severe.