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 post-transfusion hepatitis, as judged by the transaminase levels, th 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.