In a population-based sample of 28 individuals under the age of 20 years, autistic symptoms were present in 24 and DSM-III-R autistic disorder in 17. Many of the children and adolescents diagnosed as autistic also showed attention deficit/hyperactivity. There was no specific association between autistic behaviour and the presence of infantile spasms. Some of the children with tuberous sclerosis and autism were of near-normal intelligence. Indirectly, the results suggest that as many as 9 per cent of all children with autism may have tuberous sclerosis.