We performed a follow-up study on 70 patients with acute non-A, non-B (NANB) posttransfusion hepatitis and a retrospective study on 283 chronic hepatitis, 70 cirrhosis and 53 hepatocellular carcinoma patients of type NANB. In acute NANB posttransfusion hepatitis, as judged by the transaminase levels, the duration of the disease exceeded 6 months in 46/70 = 65.7% and 1 year in 32/70 = 45.7%. The histological diagnosis of the 32 cases persisting for more than 1 year was chronic active hepatitis in 5, chronic persistent hepatitis in 2 and unresolved hepatitis in 6. The frequency of previous transfusion in chronic hepatitis, cirrhosis and hepatocellular carcinoma of type NANB was 42.8, 37.1 and 15.1%, respectively, whereas the incidence of early posttransfusion hepatitis was 8.5, 8.6 and 7.5%, respectively. In chronic liver diseases with a history of jaundice and/or hepatitis, previous transfusions are more frequently associated with type NANB than with type B disease. The present study demonstrates that NANB posttransfusion hepatitis tends to develop to chronic liver disease when analyzed prospectively as well as retrospectively.