Until recent years, the forms of porphyria were classified on clinical grounds alone. The photocutaneous symptoms common to G眉nther's congenital and chronic forms1 have often led to confusion. Studies in this laboratory have revealed that the true porphyria congenita of G眉nther is an erythropoietic disease in which the site of abnormal porphyrin formation is the developing normoblast.2-4 In contradistinction, the chronic, or "cutanea tarda," variety belongs in the category of hepatic porphyria, the fundamental disturbance being that of a great overproduction of porphyrin by the liver.2,3 Human erythropoietic porphyria is a rare disease; in our own roster only 7 cases are represented in a total of 255, the remainder belonging to one of the hepatic categories, either intermittent acute, cutanea tarda, or combined types. Schmid and coworkers4 found only 34 bona fide cases in the world's literature up to 1955 but pointed out that many cases of "cutanea tarda" type