The EEG findings in waking, sleep and sphenoidal electrode recordings in 96 patients with partial epileptic seizures with complex symptoms, who, after a median interval of 18 yr developed paranoid/hallucinatory psychosis, were compared with the findings from a group of patients without psychosis, who had had the same type of epilepsy in median 24 yr. There were no significant differences between the 2 groups with regard to median age at onset of epilepsy or complex partial seizures, age or duration of epilepsy at time of examination. The psychotic patients had a significant preponderance of temporal medio-basal spike foci, recorded on the sphenoidal electrode, indicating deep temporal lobe of dysfunction as an important factor in the pathogenesis of psychosis. A significantly higher frequency of bilateral and multiple spike foci, together with a significant frequency of slow-wave admixture to the waking background EEG activity, indicated more extensive and severe epileptogenic lesions in the psychotic patients. There was no correlation between psychosis and unilateral EEG-foci in either temporal lobe.