Four patients with abrupt onset of psychosis had prominent periodic EEG abnormalities with diffuse sharp waves. Other laboratory studies, including CSF and CT, were normal. The EEG abnormalities disappeared within 1 to 7 days. Clinical improvement was complete and commenced after the EEG showed improvement. The cause remains obscure, but it seems to be a transient organic disturbance of cerebral function that must be distinguished from primary psychiatric disorders.