Thirty-eight patients with porphyria cutanea tarda (PCT) have been seen in the last 18 years. Five of these patients (13%) developed hepatocellular carcinoma (HCC) during follow-up. We analyzed the differences in clinical, laboratory and liver histology findings at presentation, between patients who developed HCC during follow-up (HCC-group, n = 5) and those who did not (PCT-group, n = 33). Of the clinical features the duration of skin-symptoms was longer in the HCC-group (mean: 10.4 +/- 1.1 years) than in the PCT-group (mean: 1.4 +/- 1 years) (p less than 0.001). No differences in routine laboratory findings were found. Although 11/38 (29%) patients had serologic evidence of a past hepatitis B virus infection and 7/38 (18%) patients had antibodies against hepatitis C virus, no differences in these parameters were found between the PCT-group and the HCC-group. In all 34 liver biopsies a variable degree of siderosis was found (PCT-group vs. HCC-group: NS). Only piecemeal necrosis (p less than 0.01) and advanced fibrosis or cirrhosis (p less than 0.001) were more common in liver biopsies in the HCC-group. In conclusion, factors related to an increased risk of HCC in PCT are: a) a long symptomatic period before start of treatment and b) the presence of chronic active hepatitis and/or advanced fibrosis or cirrhosis in liver biopsies.