A total of 149 children aged 13-17 years were examined. 83 were mildly and 66 severely mentally retarded. These children, especially the severely retarded ones, are representative of all mentally retarded children born in 1966-1970 and living in G√∂teborg, Sweden. 64% of the severely mentally retarded and 57% of the mildly mentally retarded children were suffering from a handicapping psychiatric condition. Autism-like 'psychotic behaviour' was common in the severely retarded. 0.2% of the total child population aged 13-17 years suffering from the combination of mental retardation and 'psychotic behaviour'. Epilepsy was associated with psychiatric abnormality, but Down's syndrome was generally not so associated.