The epidemiology of autism in adults has relied on untested projections using childhood research.

To derive representative estimates of the prevalence of autism and key associations in adults of all ages and ability levels.

Comparable clinical diagnostic assessments of 7274 Adult Psychiatric Morbidity Survey participants combined with a population case-register survey of 290 adults with intellectual disability.

The combined prevalence of autism in adults of all ages in England was 11/1000 (95% CI 3–19/1000). It was higher in those with moderate to profound intellectual disability (odds ratio (OR) = 63.5, 95% CI 27.4–147.2). Male gender was a strong predictor of autism only in those with no or mild intellectual disability (adjusted OR = 8.5, 95% CI 2.0–34.9; interaction with gender, P = 0.03).

Few adults with autism have intellectual disability; however, autism is more prevalent in this population. Autism measures may miss more women with autism.