The prevalence and the clinical significance of hepatitis C (HCV) infection in recipients of kidney transplantation was assessed using second generation anti-HCV antibody (anti-HCV-2). Out of 88 patients whose preoperative sera were available for anti-HCV-2 determination, 27 patients (30.7%) were positive. Transfusion units were significantly larger and the duration of hemodialysis was significantly longer in the patients with anti-HCV-2 than those without. In 10 patients whose preoperative sera were negative for anti-HCV-2, seroconversion was documented after the operation, so that the postoperative positive rate of anti-HCV-2 increased to 42.9% (39/91). Seroconversion from positive to negative was observed in only one patient. Out of 91 patients who were followed-up at least 3 months after operation, 66 patients (72.5%) developed liver dysfunction. According to the criteria of non-A, non-B post-transfusion hepatitis established by the Japanese Society of Digestive Disease, 31 of 66 patients (34.1%) were diagnosed as "definite", 21 patients (23.1%) as "suspicious". The anti-HCV-2 positive rate was 90.3% in the "definite" group, which was significantly higher than the other groups. Liver dysfunction in the patients with anti-HCV-2 had a tendency for a chronic or prolonged course. Out of 20 patients in whom liver dysfunction continued for more than 1 year, 18 patients were positive for anti-HCV-2. It is concluded from this study that the prevalence of hepatitis C is very high in kidney transplant recipients with HCV as the main and most important etiologic factor of liver dysfunction, especially in chronic liver impairment.