The manifestation of porphyria cutanea tarda reflects genetic and environmental factors. Mutations in the uroporphyrinogen decarboxylase gene, located at chromosome 1p34, discriminate familial porphyria cutanea tarda from sporadic cases. Furthermore, mutations in the haemochromatosis gene may be involved in the aetiology. In this study 53 unrelated Danish patients with porphyria cutanea tarda were classified according to uroporphyrinogen decarboxylase and haemochromatosis gene mutations and the genotype related to the clinical and biochemical data. Thirteen patients (25%) had familial porphyria cutanea tarda. The results signify the advantage of DNA diagnostics for identification of familial cases, as anamnestic data are doubtful and erythrocyte uroporphyrinogen decarboxylase activity measurements insufficient for correct classification. Eight patients with porphyria cutanea tarda (15%) were homozygous for the haemochromatosis gene C282Y mutation and 8 patients were heterozygous. Patients homozygous for the haemochromatosis related mutation showed biochemical evidence of excessive iron storage as well as increased urine porphyrin excretion levels. This seems to confirm a relationship between porphyria cutanea tarda and haemochromatosis. No differences were found between patients with sporadic and familial porphyria cutanea tarda regarding age of onset, clinical severity, sex distribution, liver function tests and iron storage parameters. However, daily alcohol intake and use of oestrogens were reported more frequently in the group of sporadic patients. It was found that women were over-represented in our study.