Serum markers of hepatitis B virus (HBV) infection were determined in 82 patients with porphyria cutanea tarda (PCT). Pathogenetic factors (alcohol, thalassemia minor, drugs) and clinical and histologic findings of PCT were taken into account. The prevalence of HBV infection was very high (70.7%). Hepatitis B surface antigen (HBsAg) was positive in 14 patients (17%). Eight patients had HBV infection as the only documented acquired factor. The clinical picture and histologic findings were aggravated by HBV infection; primary hepatic carcinoma occurred in four patients with HBV infection. Liver siderosis was histologically documented in 82.6% of cases, serum ferritin was pathologically increased in 91%, confirming the role of iron overload in PCT. A correlation (p less than 0.02; chi-squared method) was found between increased serum ferritin levels and HBV infection, suggesting a possible relationship between liver siderosis and HBV clearance. HBV infection appears to be a relevant additional factor in the pathogenesis of PCT liver disease.