Behavioural changes have often been noted in patients with epilepsy. This study investigated the converse phenomenon—the occurrence of transient sensory, cognitive and affective changes resembling those described by epileptics, in affectively ill patients. Forty-four patients with affective illness, 37 with complex partial seizures, and 30 hypertensive controls were interviewed to determine the lifetime occurrence of these phenomena. Such symptoms occurred frequently in association with episodes of affective illness and epilepsy, but were rare in controls. Visual, auditory, olfactory and epigastric symptoms, illusions, jumbled thoughts and amnesia were common to both epilepsy and affective illness. Greater numbers of symptoms were associated with better response to lithium and tricyclic antidepressants. Transient sensory, cognitive, and affective phenomena may be more common in affective illness and other psychiatric conditions than is generally recognised, and may be clues to the underlying pathophysiology of these conditions.