The aim of this study was to examine the prevalence of angelman syndrome in prepubertal school-aged children and analyze its comorbidity with autistic disorder. A clinical/psychiatric evaluation of a population-based sample of 6- to 13-year-old mentally retarded children with active epilepsy was performed. Four individuals in a total population of almost 49,000 children conformed to the clinical diagnosis of Angelman syndrome. Two of these had a typical microdeletion at chromosome 15q11-13. The minimum prevalence of Angelman syndrome was estimated at 0.008% (1: 12,000) in the examined age group. All 4 children with Angelman syndrome met full behavioral criteria for the diagnosis of autistic disorder/childhood autism. It is concluded that Angelman syndrome is uncommon, but more frequent than previously estimated. The diagnosis should be considered in all patients with combined autistic disorder, severe mental retardation, and epilepsy. The implications of the possible association of Angelman syndrome and autism are discussed.