In previous studies, low amplitudes and asymmetrical topography with right-sided peaks of the P300-evoked response have been repeatedly described in schizophrenic patients. A total of 18 patients with cycloid psychosis fulfilling the criteria of Perris and Brockington and 18 controls were investigated with a standard auditory odd-ball paradigm and multichannel-evoked potential recordings. Patients had normal P300 topographies and latencies but significantly higher amplitudes than controls. Higher than normal P300 amplitudes have not been described in any other psychiatric disorder until now, and indicate an enhanced level of arousal. Future studies are expected to shed light on the question of whether high P300 amplitudes are transitory sequelae of the acute psychotic episode or a trait of cycloid psychosis.