One hundred thirty-one patients followed at the New England Hemophilia Center (Worcester, MA) were tested for antibody to hepatitis C virus (HCV). All but two had used factor concentrate that had not undergone viral inactivation; two patients had used only cryoprecipitate. The overall prevalence of HCV antibody positivity was 76.3%. There was no significant difference in age or the amount of non-heat-treated factor concentrate used between the group that was HCV antibody positive and negative. There was also no significant difference between aminotransferase levels in the two groups. There was a positive association between HCV antibody and the presence of antibody to hepatitis B core antigen and antibody to human immunodeficiency virus. A group of 31 patients were tested twice for HCV antibody at intervals of 35 to 71 months. In this subset, 25 were repeatedly seropositive, 4 were repeatedly seronegative, and 2 went from seropositive to seronegative. These data confirm the previous impression that non-A, non-B hepatitis is a major sequela to the use of pooled coagulation factor concentrates. HCV infection may account for most of the chronic liver disease observed in this population. Anti-HCV testing of plasma donors and improved methods of viral inactivation should prevent new cases from developing.