Porphyria cutanea tarda (PCT) is the most common of the porphyrias and is due to a marked deficiency of uroporphyrinogen decarboxylase in liver. Unlike the deficiencies of heme pathway enzymes found in other porphyurias, which are thought to be exclusively genetic in origin, the uroporphyrinogen decarboxylase deficiency found in PCT is primarily acquired. PCT is also the most readily treated porphyria, and it is of interest that the two major treatments-repeated phlebotomy and low dose chloroquine appear to act by different mechanisms.