The prevalence of antibody to hepatitis C virus (HCV) was studied in 207 patients with chronic liver disease of unknown etiology, in relation to clinical, epidemiological and histological features. Serum antibody to C-100 epitope of HCV was detected by ELISA in 82.6% of patients, with a significant difference compared with a group of patients with primary biliary cirrhosis (10%). The presence of anti-HCV antibody in serum did not correlate with age, sex, histological diagnosis, and activity and duration of the disease, nor with serum anti-HBc, used as a marker of exposure to hepatitis B virus infection. These results strongly support the view that most cases that were previously defined as cryptogenic forms of chronic liver disease are in fact related to HCV infection. There was a correlation between serum anti-HCV antibody and history of risk for parenteral exposure or of acute hepatitis. This correlation was particularly evident for transmission by parenteral route, suggesting that HCV infection may be transmitted often by this route (36.8% among anti-HCV antibody-positive patients and 11.1% among anti-HCV-negative patients). Liver disease in patients without risk factors for parenteral transmission and with lower prevalence of anti-HCV antibody may be caused by other as yet unidentified non-A, non-B (non-C) agents or may be of nonviral origin.