The authors screened 3,867 psychiatric inpatients for intermittent acute porphyria by use of a spot test to detect diminished activity of the erythrocyte enzyme porphobilinogen (PBG) deaminase. Eighteen individuals so identified also had persistently diminished quantitative activity of PBG deaminase. Eight of these appeared to have intermittent acute porphyria by the added criteria of increased urinary delta-aminolevulinic acid or PBG or a family history of intermittent acute porphyria. The overall prevalence of intermittent acute porphyria was 0.21%, a considerably higher rate than that in the general population. Most of the subjects with the disorder had periods of agitated psychosis and apathetic or depressed withdrawal, with signs of neuropsychological impairment. Neurologic abnormalities were not prevalent.