We examined the stability of cognitive and adaptive behaviour standard scores in children with autism spectrum disorder (ASD) between diagnosis and school entry approximately age 6. IQ increased 18 points in 2-year-olds, 12 points in 3-year-olds, and 9 points in 4-year-olds (N = 281). Adaptive behaviour scores increased 4 points across age groups (N = 289). At school entry, 24 % of children met criteria for intellectual disability (cognitive and adaptive behaviour scores <70). No children with both scores ≥70 at diagnosis later met criteria for intellectual disability. Outcomes were more variable for children with initial delays in both areas (in 57 %, both scores remained <70). Findings are relevant to clinical decision-making, including specification of intellectual disability in young children with ASD.