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Congenital erythropoietic porphyria is a rare genetic disorder in which deficiency of uroporphyrinogen III synthase results in excessive production of Type I porphyrins. The main clinical features are severe photodestruction of the skin and haemolytic anaemia. Treatment consists of shielding from light, blood transfusions and splenectomy, but is generally unsatisfactory. Previous studies have suggested that oral charcoal may be of benefit by binding porphyrins in the gut. A trial was therefore undertaken to evaluate this possibility. Porphyrins in urine, plasma and erythrocytes were measured by HPLC in a 23-year-old male patient with congenital erythropoietic porphyria, during an 8 week "run-in" period, and for a further 3 weeks when oral charcoal was given. Total urinary porphyrin excretion was 79-283 mumol/24 h consisting of 75% uroporphyrin I, 15% coproporphyrin I and smaller amounts of hepta-, hexa-, and pentacarboxylic porphyrins. Similar proportions were found in plasma and erythrocytes. During the first 24 h of charcoal administration a minor decrease in plasma and erythrocyte porphyrins was detected but this was not maintained during the remainder of the trial. In bile and faeces coproporphyrin I constituted approximately 95% of the porphyrins, with 2-3% coproporphyrin III and smaller amounts of pentaporphyrins I and III, but only trace amounts of uroporphyrin I. Oral charcoal was of no value in this case. Reasons are discussed in the context of biochemical differences between this patient with classical Gunther's disease and the similar clinical syndrome due to deficiency of uroporphyrinogen decarboxylase.