In a record-linkage study in Stockholm, Sweden, the year 2011 prevalence of diagnosed autism spectrum disorders (ASD) was found to be 0.40, 1.74, 2.46, and 1.76 % among 0–5, 6–12, 13–17, and 18–27 year olds, respectively. The corresponding proportion of cases with a recorded diagnosis of intellectual disability was 17.4, 22.1, 26.1 and 29.4 %. Between 2001 and 2011, ASD prevalence increased almost 3.5 fold among children aged 2–17 years. The increase was mainly accounted for by an eightfold increase of ASD without intellectual disability (from 0.14 to 1.10 %), while the prevalence of ASD with intellectual disability increased only slightly (from 0.28 to 0.34 %). The increase in ASD prevalence is likely contributed to by extrinsic factors such as increased awareness and diagnostics.