Acute intermittent porphyria is an inborn error of metabolism characterized by the excretion of excess porphyrin precursors (porphobilinogen and usually delta-aminolevulinic acid) in the urine, and by sporadic attacks of neurologic dysfunction. The disease is complex, involving variable patterns of autonomic and peripheral neuropathy as well as the central nervous system manifestations. There may be alterations in carbohydrate, lipid, water, and electrolyte metabolism in addition to clinically inapparent endocrine abnormalities. The fundamental defect is thought to be a 50% decrease of uroporphyrinogen I synthetase, the third enzyme of the heme biosynthetic pathway. This is associated with a marked increase of hepatic delta-aminolevulinic acid synthetase, the first and rate controlling enzyme of the pathway. The measurement of uroporphyrinogen I synthetase in erythrocytes now provides an enzyme diagnostic test for the disease. Two therapeutic approaches that may prove to reverse the fundamental disease process, at least in some patients, involve [1] a high carbohydrate intake, and [2] intravenous administration of hematin. The latter, only recently introduced, is now being investigated.