Introduction Porphyria is thought to be an "inborn error of metabolism" (Garrod, 1923), i. e., of pyrrole metabolism. Quite a number of classifications of porphyria have been described, but among them the best known are those of Waldenström (1937) and of Schmid, Schwartz, and Watson (1953, 1954). In order to avoid any confusion the classifications of Günther (1922), Micheli and Dominici (1931), Discombe (1948), Prunty (1952), and Casalis (1954) are not to be discussed in this paper. Waldenström divided porphyria into three types: porphyria congenita, porphyria cutanea tarda (P. C. T.), and porphyria acuta (with clinical subdivision). He distinctly separated the congenital form from the others, as this type was never found in the families in which other types were seen. This type has a marked photosensitivity, resulting in severe mutilations very frequently accompanied by hemolytic anemia and splenomegaly. Waldenström designated P. C. T. as the form