A new form of acute hepatic porphyria with double genetic defect--deficiency of porphobilinogen deaminase and coproporphyrinogen oxidase--is described. Among 17 studied family members this double enzymatic deficiency was found in five individuals, deficiency of porphobilinogen deaminase in four, and deficiency of coproporphyrinogen oxidase in two. Only the proband had an attack of porphyria. Apart from the proband, all family members had normal urinary PBG excretion. Increased faecal coproporphyrin excretion was found in three people. The results obtained suggest that deficiency of porphobilinogen deaminase and coproporphyrinogen oxidase can be inherited independently. coproporphyrinogen oxidase can be inherited independently.