The prevalence of antibodies against hepatitis C virus (HCV) was investigated in 96 patients with hepatocellular carcinoma, 106 patients with liver cirrhosis without evidence of cancer, and 177 controls without liver disease. 75% of patients with hepatocellular carcinoma had HCV antibodies (anti-HCV), a significantly higher proportion than that observed in patients with cirrhosis (55·6%), or controls (7·3%). The prevalence of anti-HCV was significantly higher in patients with alcoholic cirrhosis and hepatocellular carcinoma (76%) than in patients with alcoholic cirrhosis alone (38·7%), whereas in patients with cryptogenic cirrhosis there was no significant difference between those with and without primary liver cell cancer (81·4% and 77·5%, respectively). These results indicate that HCV infection may have a role in the pathogenesis of hepatocellular carcinoma, even in patients with chronic liver disease apparently related to other agents such as alcohol, and that this recently identified hepatitis virus may be found in a large proportion of patients with cryptogenic cirrhosis.