Recent studies of the prevalence of autism have suggested higher estimates than previously described. Various diagnostic criteria for autism and related disorders have been applied, with variability in case finding methodology and characteristics of populations as well. In this study, maternal and child health clinics covering 98% of the population were used for screening pervasive developmental disorders. Extensive medical investigation was carried out on the majority of cases. In this Norwegian population of children ages 3-14 years the minimum prevalence estimate for childhood autism was 4-5 per 10,000 using ICD-10 research criteria, and did not confirm the high estimates suggested more recently. Medical disorders identified were associated with mental retardation rather than specifically with autism.