A population-based study of active epilepsy was conducted in 6-13-year-old mentally retarded children born between 1975 and 1986. The population at risk comprised 48,873 children. Ninety-eight children were identified, 35 mildly and 63 severely retarded. The prevalence was 2.0 per 1000; 0.7 per 1000 for mildly and 1.3 per 1000 for severely retarded children. Sixty-nine children had at least one additional neuroimpairment. Cerebral palsy was found in 42 children with a majority of spastic/dystonic tetraplegias; visual impairment was present in 24 and autism in 24. Thirty-three children had only a mild or no gross motor disability and mild mental retardation, while 23 had IQs < 20 and a very severe gross motor disability. This study underlines the fact that active epilepsy in mentally retarded children is often associated with additional neuroimpairments, especially a combination of severe cerebral palsy and severe visual impairment.