To assess the epidemiology of hepatitis C virus (HCV) in Poland, anti-HCV was studied in patients with acute and chronic non-A, non-B (NANB) hepatitis, in healthy adults, and in subjects at risk. Anti-HCV prevalence was 2% in 152 blood donors, 78% in 95 parenteral drug addicts, 21% in 112 alcoholics, and 86% in 42 patients with chronic NANB hepatitis. Among 34 prospectively followed patients with acute NANB hepatitis 17 (50%) developed anti-HCV. It seems that HCV infection is responsible for the majority of NANB hepatitis in Poland and is common in parenteral drug abusers and alcoholics.