A 36-year-old white man had both acute intermittent porphyria and long-standing idiopathic grand mal seizures. Diphenylhydantoin apparently adversely affected both the clinical and biochemical parameters of the acute intermittent porphyria. Comparison of urinary levels of the porphyrin precursors, delta aminolevulinic acid and porphobilinogen, under controlled diet conditions before and after withdrawal of diphenylhydantoin, showed that this drug accounted for approximately one-half of the porphyrin precursor excretion. Significant clinical improvement of the porphyria followed withdrawal of the diphenylhydantoin. Bromides appeared to be approximately as effective as diphenylhydantoin for seizure control in this patient.