Antibody to the recently identified hepatitis C virus was investigated in sera of 128 patients treated with allogeneic bone marrow transplantation, to determine the prevalence of HCV infection and its role in post-transplant liver complications. The overall prevalence of anti-HCV positivity was 28.6% (38/128 patients). The presence of pretransplant anti-HCV positivity (in 10/35 tested patients) did not seem to predict a more severe liver disease. In fact 8/10 anti-HCV+ and 15/25 anti-HCV- patients had elevated transaminases at BMT, and posttransplant liver failure (due to VOD or subacute hepatitis), and post-BMT rises in transaminases occurred regardless of anti-HCV serology (P = 0.6 and 0.2, respectively). In patients tested for anti-HCV after BMT (n = 128), only two (one anti-HCV+ and one anti-HCV-) experienced VOD; the number of patients in whom liver failure contributed to death was comparable in anti-HCV-positive and anti-HCV- negative patients (P = 0.4). Among 17 patients with documented posttransplant seroconversion (from anti-HCV- to anti-HCV+) the appearance of anti-HCV was concomitant with hepatitis exacerbation in 9 (53%). Histologic changes demonstrated a more severe liver damage in anti-HCV+ patients: a chronic hepatitis was diagnosed in 9/11 anti-HCV+ versus 1/7 anti-HCV- cases. Based on these observations, we conclude that hepatitis C virus has a role in liver disease in such patients, although its evaluation by the anti-HCV test is still of limited accuracy, due to low sensitivity and incomplete specificity.