Observed prevalence of autism spectrum disorders in two Norwegian counties.

The prevalence of autism spectrum disorders (ASD) has previously been reported to be increasing dramatically in European and non-European countries. No similar increase in prevalence rates has been documented in Norway to date. The current study reports on ASD prevalence rates in two Norwegian counties.The population comprised 31,015 children, ages six to 12. Information about special needs services was provided by the school authorities and the public health service. Multiple search strategies were applied to identify children at risk of ASD or diagnosed with ASD. Hospital registers were searched and a mapping tool was used in all local schools.The total number of patients with ASD found in the population was 158. This gives a prevalence of 51 per 10,000 (95% CI, 0.43-0.59).Compared with the previously reported prevalence of ASD in Norway, there has been almost a fourfold increase in the occurrence of childhood autism and a tenfold increase in the occurrence of all ASD groups. These findings have significant implications for designing and dimensioning appropriate intervention programmes for children with ASD and their families.