The frequency of hepatitis C virus (HCV) infection in Spain was assessed by means of a recombinant-based immunoassay for serum anti-HCV antibodies. 836 serum samples were tested from 676 patients selected according to their risk of blood-borne viral infections and presence of liver disease. Among patients at high risk of infection (with or without liver disease) anti-HCV antibodies were found in 85% of prospectively followed patients with post-transfusion non-A, non-B hepatitis, 62% of patients with chronic hepatitis or cirrhosis and a history of blood transfusion, 70% of haemophiliacs receiving replacement therapy, 70% of intravenous drug abusers, and 20% of haemodialysis patients. Only 8% of homosexual men infected with human immunodeficiency virus and 6% of female contacts of drug abusers were positive. Among patients with liver disease and no history of parenteral exposure to blood, anti-HCV antibodies were detected in 38% with cryptogenic, alcoholic, or primary biliary cirrhosis and in 44% with chronic active hepatitis. Among healthy subjects without risk factors for hepatitis the overall prevalence of anti-HCV was 1·2%. © 1989.