A follow-up study to at least the age of 21 years of 135 individuals with an autism spectrum disorder diagnosed in childhood and an IQ of over 30 was conducted. The study is distinctive in its large size, low attrition rate and use of systematic interviews to obtain clinical information. Questionnaires completed by caregivers asked about the development of new psychiatric disorders. For the 39 individuals with a possible new disorder, a detailed psychiatric assessment was undertaken through parental interview. Of all participants, 16 percent developed a definite new psychiatric disorder. A further 6 percent developed a possible new disorder. Five individuals developed an obsessive-compulsive disorder and/or catatonia; eight an affective disorder with marked obsessional features; three complex affective disorders; four more straightforward affective disorders; one a bipolar disorder; and one an acute anxiety state complicated by alcohol excess. There was no case of schizophrenia.