Nine of 33 patients diagnosed as functional psychosis, who had negative neurological examinations and laboratory studies, showed gross pathological changes of the vermis when the cerebellum was exposed for implantation of a vermal stimulator. These findings motivated us to review the CAT scans done on 264 psychiatric patients as part of their work-up to rule out organic or structural aberration. Although 132 (50 per cent) of these scans were reported as abnormal, only 17 showed pathology of the cerebellum. With careful rereading with attention to the posterior fossa, however, the incidence of cerebellar vermal abnormality proved to be much higher. Of the 96 patients in the series who were diagnosed as functional psychosis, 31 showed pathology of the cerebellar vermis (22 of 65 schizophrenics and 9 of 31 patients with other functional psychosis). Scans of seven patients who had been diagnosed and treated as functional psychosis revealed tumors of the vermis. Scans were then obtained on 20 additional patients with a diagnosis of schizophrenia. These patients were selected at random from two psychiatric services, excluding only those patients who had findings suggesting organicity or who had histories of alcoholism or electroconvulsive therapy. Twelve of the 20 patients (60 per cent) showed atrophy of the vermis of the cerebellum. Thus a total of 34 of 85 schizophrenic patients (40 per cent) had scans showing pathology of the cerebellar vermis. These data are consistent with data from animal experiments that demonstrate a role for the cerebellar vermis in behavioral pathology and epilepsy.