The efficacy of hematin has been evaluated in eight patients with acute intermittent porphyria: six with acute attacks and two with chronic subacute symptoms. Hematin suppressed the chemical signs of porphyria in all patients and the symptoms in those with acute attacks. The clinical response to hematin occurred uniformly on the third or fourth day of treatment, suggesting that a stereotypic response to this form of therapy can be defined. Hematin had no effect on chronic subacute symptoms. The indications for hematin therapy, its rationale and the details of its administration are reviewed.