There is usually a causal relationship between epilepsy and mental retardation when they coexist. The pathogenetic period of the underlying brain disorder and the time of seizure onset may, however, be widely separated. In 63 institutionalized mentally retarded epilepsy patients, 41% had seizure onset prior to the age of 2, 30% between the age of 2 and 20, and as many as 29% after the age of 20. Whereas uncontrolled epilepsy and cerebral palsy were frequently present in the group of early onset, most patients with adult onset had well controlled epilepsy, and none had cerebral palsy. Five of the 18 patients with adult onset epilepsy had Down's syndrome; in three of them, concomitant dementia was evident. In most patients with adult onset, however, no recent or current etiological factors for seizures were evident, except for their stable intellectual deficits. The incidence of adult onset epilepsy in the institution in 10 years markedly exceeded the expected rate. Twenty-three patients had achieved a prolonged remission of seizures and discontinued antiepileptic treatment. Sixty per cent of them remained seizure-free after at least two years. The mentally retarded should not unduly be subjected to drugs with potential central nervous side-effects.