We have followed up 69 patients who developed non-A, non-B posttransfusion hepatitis in 1972-1978. Chronic hepatitis, defined by biochemical criteria, was observed in 46 patients (67%), the majority of whom subsequently failed to resolve the abnormalities. Chronic hepatitis was a sequela of non-A, non-B posttransfusion hepatitis less often after the blood bank changed to a policy of all volunteer donors. (However, this association may be explained by other coexistent factors.) The alanine aminotransferase level was more likely to be abnormal than the aspartate aminotransferase level during the chronic phase of non-A, non-B posttransfusion hepatitis. By actuarial means it was calculated that the probability of developing normal enzymes after 6-10 yr was 0.47. However, in spite of this high incidence of biochemical disease, virtually all of the patients have remained asymptomatic. Histologic evidence of cirrhosis has been obtained in 4 of these patients, but in only 2 patients at most has clinical evidence of hepatic failure supervened.