Two community-based cohorts of children with autism spectrum disorder, examined using similar assessment protocols, were pooled (n?=?301) and subdivided according to history of regression. Those with regression (n?=?62), 20.5% of the combined cohort, were contrasted with those without regression (n?=?241) at first assessment (age range 19-60 months) and at 2-year follow-up on a range of measures. The regression group was significantly more functionally impaired, with regard to intellectual function (p?