There are few long-term follow-up studies of children with Autism Spectrum Disorders (ASD) who attended intensive intervention programmes in their pre-school years. Thirty-six children with ASD enrolled in relatively intensive, specialist pre-school programmes (minimum of 15 h intervention per week for 2 years at a mean age of 3.4 years) were assessed after 2 years (mean age 5.5 years) and again after a further 5 years (mean age 10.3 years). Cognitive, language and adaptive behaviour skills and severity of autism symptoms were assessed at intake (Time 1) and subsequent follow-ups (Times 2 and 3). Children made significant increases in raw and age equivalent scores in most areas of development assessed, although mean standard scores remained stable or decreased over time. Time 1 IQ, language and adaptive behaviour skills were predictive of outcome at Time 3. Although there were marked individual differences in the rate and patterns of change over time, many children continued to show increases in test scores over the course of the study. This study highlights that whilst overall group improvements may be evident, the rate and nature of these improvements is highly variable across individual children. Further investigation of the specific child characteristics that affect treatment effectiveness is required. (Contains 3 figures and 4 tables.)