We have evaluated the clinical and histopathological outcomes of patients who contracted chronic non A, non B hepatitis as a result of transfusions administered during heart surgery at the National Institutes of Health. Posttransfusion hepatitis developed in 65 of 1,070 (6.1%) patients and became chronic in 45 (69%) of those cases. Antibody to hepatitis C virus was detectable in 53 patients (82%) with posttransfusion non A, non B hepatitis. Thirty-three patients with chronic non A, non B hepatitis agreed to liver biopsy (group 1). In addition, six other patients with chronic posttransfusion non A, non B hepatitis were evaluated (group 2). These 39 patients were followed between 1 and 24 yr (mean = 9.7 yr). Cirrhosis developed in 8 patients (20%) between 1.5 and 16 yr after blood transfusion. Of the 33 patients in group 1, 11 (33%) died during follow-up. In two cases (6%), this was related to liver failure. At this writing, two additional patients (6%) have decompensated cirrhosis and one (3%) had debilitating fatigue. Twenty of 33 patients (61%) with histological evidence of chronic active hepatitis or cirrhosis are asymptomatic and have no clinical evidence of liver disease. Thus chronic non A, non B posttransfusion hepatitis appeared to be due to hepatitis C virus infection in most cases. It was associated with the development of cirrhosis in approximately 20% of cases and end-stage liver disease in 12% of patients followed prospectively. Most patients with histological evidence of cirrhosis or chronic active hepatitis, however, had minimal clinical evidence of liver disease within the time frame of this study.