Aims: To determine the prevalence of autistic spectrum disorder in a geographi cally defined population for children receiving services and compare this to the estimated prevalence based on a two source “capture-recapture”technique prev iously employed in biological populations to give a “true”prevalence with full ascertainment. Methods: Information on gender, age, and postcode sector was det ermined from nine different datasets maintained for children with autistic spect rum disorder and point prevalence was calculated. Data from the diagnostic servi ces and the Special Needs System were entered into the two source “capture-rec apture”calculation. Results: Of a total population of 134 661 under 15 year old s resident in Lothian in southeast Scotland, 443 were known to autism services, with a point prevalence of 32.9 per 10 000 (95%CI 29.8 to 36.0). The estimated prevalence using a capture-recap- ture method was 44.2 (95%CI 39.5 to 48.9), which suggests that 74%of affecte d children were registered with services in some way. The age distribution was s imilar to that of the background population under the age of 12 years and there was no indication of a rising prevalence. The ratio of boys to girls was 7:1. Co nclusions: The prevalence of autistic spectrum disorder in a geographically base d population employing two source capture-recapture analysis is comparable to t hat quoted for the best active ascertainment studies. This technique offers a to ol for establishing the prevalence of this condition in health service populatio ns to assist in planning clinical services. Aims: To determine the prevalence of autistic spectrum disorder in a geographic an cally defined population for children receiving services and compare this to the estimated prevalence based on a two source “capture-recapture” technique prev iously employed in biological populations to give a “true” Methods: Information on gender, age, and postcode sector was det ermined from nine different datasets maintained for children with autistic spect rum disorder and point prevalence was calculated. Data from the diagnostic servi ces and the Special Needs System were entered Results: Of a total population of 134 661 under 15 year old s resident in Lothian in southeast Scotland, 443 were known to autism services, with a point prevalence of 32.9 per 10 000 ( 95% CI 29.8 to 36.0). The estimated prevalence using a capture-recapture method was 44.2 (95% CI 39.5 to 48.9), which suggests that 74% of affecte d The age distribution was s imilar to that of the background population under the age of 12 years and there was no indication of a rising prevalence. The ratio of boys to girls was 7: 1. Co nclusions : The prevalence of autistic spectrum disorder in a geographically base d population employing two source capture-recapture analysis is comparable to t hat quoted for the best active ascertainment studies. This technique offers a to ol for establishing the prevalence of this condition in health service populatio ns to assist in planning clinical services.