Porphyrins, their reduced congeners (porphyrinogens), and their precursors are accumulated and excreted in excessive amounts in the porphyrias because of defects in the enzymes of heme biosynthesis. The nature of these defects is being defined using biochemical and molecular biological techniques. The principal clinical manifestations in the porphyrias, photocutaneous lesions and neurological dysfunction, are linked to the biochemical abnormalities, and appropriate therapeutic interventions have accordingly been developed. The exogenous administration of metalloporphyrins and porphyrin derivatives, unlike the harmful effects of porphyrins in the porphyrias, may be of use in some clinical conditions, such as the treatment of hyperbilirubinemic states and the detection and therapy of certain cancers.