Although enzymatic defects have been identified in porphyria cutanea tarda and variegate porphyria several important controversial points remain unresolved. Hepatic uroporphyrinogen decarboxylase activity is subnormal in all patients with porphyria cutanea tarda and is presumably responsible for the biochemical derangement which characterizes the disease. Several groups have found subnormal erythrocyte uroporphyrinogen decarboxylase activity as well and have demonstrated that the defect is inherited as an autosomal dominant trait. In other studies, erythrocyte enzyme activity has been normal and no evidence of an inherited factor has been identified. These two types of patients have been called "familial" and "sporadic" cases of porphyria cutanea tarda. Whether or not the hepatic enzyme defect is inherited in all cases remains to be determined. Two groups of investigators have identified two different enzymatic defects in variegate porphyria. One group has reported subnormal activity of heme synthase (ferrochelatase) to be the defect responsible for the disease and the other has reported subnormal activity of protoporphyrinogen oxidase to be the causative factor. Which of the two defects is responsible for the biochemical abnormalities in variegate porphyria remains to be resolved.