All mainstream primary schools in Cardiff were invited in July 1998 to participate in a prevalence survey of autistic spectrum disorder. Teachers of each class filled in a questionnaire based on ICD-10 criteria for autistic disorders. The Autism Spectrum Screening Questionnaire (ASSQ) was completed on children identified with problems identified by the questionnaire. A total of 11692 children born between 1 September 1986 and 31 August 1990 were screened: 234 (2%) children were identified as requiring an ASSQ; 151 of 234 (65%) ASSQs were returned. Of the 151, 60 children (52 male, 8 female; 40%) scored 22 or more. Their notes and the involved professionals were consulted. Thirty-five children, unknown to specialist services or with complex features, required additional assessment. Seventeen children (all male) were found to be on the autistic spectrum. When the overall rubric was disentangled we found a diverse population of affected children including a handful who did not fit easily into ICD-10 classification. Correcting for incomplete ascertainment we found a minimum prevalence of 20.2 out of every 10000 (SE=4.5) for autistic spectrum disorder in this population.