An exhaustive study of erythropoietic protoporphyria in the Netherlands led to the discovery of 200 patients with this disorder in 91 families. In 46 of these families a single patient occurred. A study of parents, sibs and children led to the conclusion that the disease of erythropoietic protoporphyria is inherited as an autosomal recessive disorder. The presence of an occasional fluorescent red blood cell combined with normal protoporphyrin levels was observed in half of the children and sibs of patients and one of their parents; this phenomenon is therefore inherited as an autosomal dominant character. From an analysis of the findings in the 91 families we put forward the hypothesis of a 3-allele system.