Porphyria cutanea tarda in human beings is believed to be due to reduced hepatic uroporphyrinogen decarboxylase activity. However, extrinsic factors such as alcohol abuse and drug intake are required for clinical manifestation of the disease. In addition to typical cutaneous lesions, patients with porphyria cutanea tarda usually have chronic liver disease and moderate iron overload. Of 74 Italian patients with porphyria cutanea tarda, hepatitis C virus antibodies were detected in 76% by enzyme-linked immunoassay and in 82% by recombinant immunoblot assay. Viral genome, studied with nested polymerase chain reaction, was found in the sera of 49 subjects--47 positive and 2 indeterminate on recombinant immunoblot assay. Five percent of the patients were HBsAg-positive, and about 40% had had past hepatitis B contacts. Alcohol abuse was present in 38%. Liver biopsies performed in 42 patients showed chronic persistent hepatitis in 7 patients, chronic active hepatitis in 22 patients, fibrosis in three patients and cirrhosis in 10 patients. Hepatitis C virus antibody was detected in 100% of patients with chronic active hepatitis and in about 80% of all other groups. Alcohol abuse was more frequent in patients with cirrhosis (80%) than in the other groups. In Italian patients with porphyria cutanea tarda, the prevalence of hepatitis C virus infection was very high, comparable to that in non-A, non-B hepatitis and high-risk patient groups. Hepatitis C virus is probably the main pathogenetic factor of the liver disease of patients with porphyria cutanea tarda.