Porphyria cutanea tarda is a disorder of porphyrin metabolism, of which familial and sporadic forms have been described. Factors such as iron seem necessary for porphyria cutanea tarda to become clinically manifest. To study the relationship between iron and uroporphyrins in hepatocytes of patients with porphyria cutanea tarda, a morphological and morphometrical study was performed in 13 liver biopsies of patients with porphyria cutanea tarda (eight with sporadic porphyria cutanea tarda and five with familial porphyria cutanea tarda). In addition, possible differences in clinical and biochemical features and in histopathological findings between patients with sporadic porphyria cutanea tarda and familial porphyria cutanea tarda were investigated. Familial porphyria cutanea tarda patients presented at a younger age than sporadic porphyria cutanea tarda patients (42.4 +/- 5.3 vs. 57.3 +/- 8.6 years). Biochemical features were not different between sporadic porphyria cutanea tarda and familial porphyria cutanea tarda patients. Uroporphyrin crystals and a variable degree of liver siderosis were detected in the biopsies of all 13 patients. Uroporphyrin crystals were often found close to ferritin-like iron deposits. The morphometrical analysis showed that an increased mean area fraction of ferritin iron was associated with an increased mean area fraction of uroporphyrin crystals in hepatocytes of sporadic porphyria cutanea tarda and familial porphyria cutanea tarda patients. Moreover, the amount of uroporphyrin crystals was significantly higher in livers of familial porphyria cutanea tarda than sporadic porphyria cutanea tarda patients. These findings are consistent with the hypothesis that uroporphyria is precipitated by an iron-dependent process.(ABSTRACT TRUNCATED AT 250 WORDS)