The recent identification of the hepatitis C virus and development of assays to detect antibodies to hepatitis C virus has allowed assessment of the prevalence of hepatitis C virus infection in patients with a variety of liver and other diseases. The aim of this study was to investigate the prevalence of hepatitis C virus antibodies and severity of liver injury in patients with porphyria cutanea tarda. Sixty-two patients were studied. Serum samples were analyzed for liver function parameters and markers of hepatitis B virus infection. Frozen serum samples from 34 patients with porphyria cutanea tarda, obtained when patients were seen at the hospital for the first time, were analyzed for hepatitis C virus antibodies with enzyme-linked immunosorbent assays (first- and second-generation) and a recombinant immunoblot assay. As controls, serum samples from 19,788 blood donors, 40 patients with alcoholic liver disease and 138 hospitalized patients without liver disease were also tested for hepatitis C virus antibodies. Liver biopsy was performed in 42 porphyria cutanea tarda patients. Specimens were evaluated for steatosis, siderosis, fibrosis, severity of inflammation and the presence of cirrhosis. In addition, the degree of necroinflammatory change and fibrosis were quantitated with the histologic activity index described by Knodell et al. The prevalence of hepatitis C virus antibodies in patients with porphyria cutanea tarda (62%) was higher than that in blood donors (0.79%), patients with alcoholic liver disease (17.5%) or hospitalized patients without liver disease (5.8%).(ABSTRACT TRUNCATED AT 250 WORDS)