The cumulative risk of seizures and epilepsy was investigated in a prospectively identified cohort of 221 children with mental retardation (MR) born between 1951 and 1955 in Aberdeen, Scotland. By age 22 years, 33 (15%) had epilepsy. An additional 16 (7%) had had at least one seizure, but did not meet the criteria for epilepsy. The cumulative risk of epilepsy was 9, 11, 13, and 15% at 5, 10, 15, and 22 years, respectively. In children with MR and no associated disabilities, the cumulative risk of epilepsy was only 2.6, 3.2, 3.9, and 5.2% at 5, 10, 15, and 22 years. In children with MR and cerebral palsy (CP), the cumulative risk was 28, 31, and 38% at 5, 10, and 22 years. Children with a postnatal injury associated with MR had a cumulative risk of epilepsy of 53, 66, and 66% at 5, 10, and 15 years after the injury. By age 22 years, 39% had achieved 5-year seizure-free remission, including 56% of children with MR without associated disability, 47% of children with MR and CP, and 11% of children with a postnatal injury. We conclude that, in the absence of associated disability or postnatal injury, the risk of epilepsy in the retarded population is low. Epilepsy in this population also will frequently enter remission in later life.