The porphyrias are disorders of heme synthesis, which has eight steps. Each type of porphyria involves a defect, either inherited or acquired, in a pathway enzyme. When the defect is physiologically significant, it results in overproduction of pathway precursors preceding the defective step which enter the circulation and are excreted into urine or bile. The diseases have been grouped as acute hepatic porphyrias and photocutaneous porphyrias. The acute porphyrias are due to hepatic overproduction of the porphyrin precursors, delta aminolevulinic acid and porphobilinogen, and the symptoms are caused by injury primarily to the nervous system. Cutaneous porphyria is due to overproduction of photosensitizing porphyrins by the liver or bone marrow, depending on the type of porphyria.