We report data on intelligence and on possibly associated psychopathology in 16 children and adolescents, between 7 and 18 years of age, with congenital or juvenile myotonic dystrophy. We found that all the subjects have an intelligence level below the population mean, four of them in the mentally retarded range. An unexpected number of subjects are in the clinical range on the Child Behavior Checklist. In nine of the 16 subjects, a child psychiatric diagnosis was found using a standardized psychiatric interview. The most frequent child psychiatric diagnosis was Attention Deficit Hyperactivity Disorder.