Objective: To compare social and behavioural outcomes between children formally diagnosed with autism spectrum disorders (ASD) with those of children who displayed autistic traits at preschool age, but remained undiagnosed as teenagers. Method: A secondary analysis of data from a birth cohort study, the Avon Longitudinal Study of Parents and Children (N = 13,944), in SW England. Children clinically diagnosed with ASD were identified from their medical records (n = 71). A comparison group, who displayed autistic traits at age 3-4, but without ASD diagnosis were also identified (n = 142). Social and behavioural outcomes in adolescence were compared between the two groups. Results: Children with ASD diagnoses were more impaired as teenagers that those in the comparison group on a range of measures of autistic-like behaviour. The developmental trajectory of prosocial behaviour showed that differences between the case and comparison groups increased dramatically in the preschool and early primary years, but that after 6 years the trajectories were similar. Conclusions: The divergence of the clinically diagnosed group and the nondiagnosed group in measures of autistic-like behaviour increased with age. This study provides evidence that it may be difficult to distinguish preschool age children who exhibit autistic-like symptoms but improve, from those who go on to develop lifelong impairment.