A patient with uncontrolled posttraumatic epilepsy and acute intermittent prophyria was subjected to successive therapeutic trials with phenytoin, carbamazepine, and clonazepam, while eating an adequate diet. Both phenytoin and carbamazepine treatments caused significant increases in porphobilinogen excretion and appeared to induce acute porphyric attacks. In contrast, treatment with clonazepam under rigid dietary control for 10 days caused no increase in porphilbinogen excretion. During the subsequent 7 months of treatment with clonazepam, neither seizures nor porphyric attacks recurred. These findings suggest that clonazepam may be a safe and effective treatment for chronic or severe generalized seizure disorders in patients with acute intermittent porphyria.