It has been clarified that hepatitis C virus(HCV) is the major cause of acute and chronic non-A, non-B hepatitis. However, it is also clear that there still exist a group of patients with viral hepatitis who are seronegative for hepatitis A, B, C, D or E virus infections. In this article, epidemiological studies on such non-A-E hepatitis in Japan are reported. Since the introduction of blood screening with hepatitis B virus(HBV) core antibody and HCV antibody in addition to HBV surface antigen, incidence of posttransfusion hepatitis decreased remarkably to a level less than 1%. Therefore, occurrence of non-A-E posttransfusion is rare in Japan. As for acute sporadic hepatitis, about 20% of the patients were diagnosed as type non-A-E. Those patients with non-A-E acute sporadic hepatitis showed relatively mild hepatitis. About 5% of patients with chronic hepatitis was negative for both markers of HBV and HCV infections. Hepatitis G virus, a newly discovered hepatitis virus, accounted for only a minor part of acute and chronic non-A-E hepatitis.