From eMedicine:
Quote:
Frequency
United States
Current estimates are that slightly more than 0.1% of the general population has classic autism by DSM-IV criteria. When the full autism spectrum is viewed, the prevalence is higher, with 0.3-0.7% having the disorder: Asperger disorder is estimated to occur in 0.2-0.5% of school-aged children, Rett disorder occurs in 5-15 girls per 100,000, and childhood disintegrative disorder occurs with a frequency of 1-4 cases in 100,000. Prevalence rates of autistic disorder in more recent studies have been noted to be as much as 40 times higher than past studies, but this does not necessarily mean that the true prevalence of the disorder is increasing. This is because more recent studies are performed in an environment of higher awareness of the disorder and use more inclusive diagnostic criteria. Remember that as recent as the 1970s, autistic children were more commonly labeled as "psychotic," thus making comparisons with this era of data highly suspect.
International
Depending upon diagnostic criteria used, rates are similar to those in the United States.
International will probably have a greater number for Asperger's, but they tend to include cases that would be deemed as PDD-NOS [and even HFA] in the US.