A prevalence study of autistic disorder (AD) in childhood was undertaken in a Welsh health district as part of a needs' assessment exercise. The study population were all children resident in the district with dates of birth between 1977 and 1988. Children were identified after a trawl of service providers in health and education and diagnosis was confirmed at a multidisciplinary assessment. An increase in prevalence was found, an observation that has been made in other parts of the world. The increase in prevalence, whether real or administratively apparent, has major implications for service providers in health, education, and social care, in that a larger number of children are being recognised as having a triad of impairments which requires very specific interventions and support from all three agencies.