Identification of the hepatitis C virus--the main cause of posttransfusion and sporadic non-A, non-B hepatitis--and the development of a diagnostic serological test have allowed us to study possible recurrence of this type of hepatitis after liver transplantation. Six of 34 consecutive transplant recipients were found to have had antibodies to hepatitis C before transplantation. All six patients had possible exposure to hepatitis C through blood transfusion or intravenous drug use. Five of the six patients were positive for antibodies to hepatitis C after 1 yr of follow-up. Two of these patients had clinical and histological evidence of acute viral hepatitis in their allografts. In one patient this led to hepatic injury and dysfunction of two successive grafts. In contrast, none of the twenty-eight patients who were seronegative for hepatitis C virus antibodies before transplantation has converted to seropositivity after transplantation despite perioperative blood transfusions. These results suggest that hepatitis C diagnosed serologically recurs in a minority of transplant recipients and that de novo seroconversion must be uncommon.