A total of 161 patients transplanted between 1978 and 1991 and who had survived at least 2 years after allogeneic bone marrow transplantation (BMT) were studied. Of 161 surviving patients, 28 (17.4%) were positive for hepatitis C virus (HCV) either by serology or polymerase chain reaction (PCR). Twenty-five patients were positive for HCV RNA by PCR, and 26 of the 28 patients had HCV antibodies detected by enzyme-linked immunosorbent assay (ELISA). The median follow-up time of HCV-positive patients was 6.1 years (range, 2.8 to 14.0 years). There was no difference in the frequency or degree of liver dysfunction between patients who were PCR-positive or -negative before BMT. Six patients developed severe liver dysfunction after BMT, and five of these patients did so after discontinuation or tapering of immunosuppression. No patient has developed liver failure. Serum transaminases were abnormal at the time of last follow up in 19 of 28 (68%) patients. Fifteen patients have had liver biopsies. No biopsy showed development of cirrhosis. We conclude that HCV is not a major contributing factor to morbidity and mortality during the first 5 to 10 years after allogeneic BMT.