ADHD and autism are generally considered separate disorders, despite recent interest in the overlapping symptoms of these disorders. This book describes a study of autistic symptoms in a group of 821 children with ADHD, combined type using the Social Communication Questionnaire. The results show that children with ADHD, combined type have more symptoms of autism than normal schoolchildren (on average). It was also shown that autistic symptoms are familial in nature, with some families having 2 children with ADHD, combined type and raised autistic symptoms while others had 2 children with ADHD, combined type and low autistic symptoms. Furthermore an association was shown between the presence of autistic symptoms in children with ADHD, combined type and the presence of oppositional defiant disorder, conduct disorder and motor co-ordination disorder. A clinical subtype of ADHD, combined type with increased autistic symptoms and an increased risk of oppositional symptoms was shown. This study has clinical implications as well as implications for research into the genetics of ADHD, the genetics of autism or the genetics of a possible overlapping neurodevelopmental disorder.