We report data on behaviour and possibly associated psychopathology in 24 children and adolescents between 9 and 22 years of age, with the childhood type of myotonic dystrophy. The patients were recruited through a patients association, as family members from patients with myotonic dystrophy (DM). In 41% of the subjects a score in the clinical abnormal range was found on the Child Behavior Checklist (completed by parents and by teachers). In 62% of the subjects, a child psychiatric diagnosis was found using a standardised psychiatric interview. The most frequent child psychiatric diagnoses were Attention Deficit with Hyperactivity Disorder and Anxiety Disorder.