We tested the sera of 29 patients treated with allogeneic bone marrow transplantation (BMT) by first- and second-generation ELISAs for hepatitis C virus (HCV) antibodies to study the effect of HCV infection on post-transplant liver diseases. Before BMT the first-generation assay detected anti-HCV in 3 of 29 patients (10%) and the second-generation assay detected anti-HCV in 5 of 29 (17%). After BMT the first-generation assay detected anti-HCV in 11 of 20 patients (55%) and the second-generation assay detected anti-HCV in 14 of 20 (70%). According to pre-transplant anti-HCV status by the second-generation assay, liver failure occurred in none of the anti-HCV-positive group and three of the anti-HCV-negative group. Graft-versus-host disease was responsible for liver failure in these patients. According to the post-transplant anti-HCV status by the second-generation assay, chronic hepatitis was found in 14 of 14 (100%) anti-HCV-positive and 1 of 6 (17%) anti-HCV-negative patients during post-transplant follow-up (p < 0.001). Post-transplant seroconversion from anti-HCV-negative to anti-HCV-positive status assay was detected by the second-generation assay in 9 of 20 (45%) patients. A biochemical deterioration during seroconversion was observed in 7 of 9 (79%) cases. HCV plays an important role in the etiology of post-transplant liver disease.