In this paper we compare adult outcome in a group of young men with autism and a group with developmental receptive language disorders. The two groups were first assessed in early childhood, when aged 7 to 8 years of age. Although matched at that time for nonverbal IQ (mean 92–93) and expressive language ability, the Autism group was significantly more impaired on most measures of social and communication skills and stereotyped behaviours. A later follow-up, in mid-childhood, suggested that although the groups were still quite distinct, social and behavioural problems had become more apparent in the Language group. The current study was completed when the participants were aged, on average, 23 to 24 years. The findings indicated that verbal IQ and receptive language scores had improved significantly more in the Autism group than in the Language group over time. Moreover, although the Language group were less severely impaired in their social use of language, many showed a number of abnormal features in this domain. There were no differences between the groups on tests of reading or spelling. Discriminant function analysis, which had clearly distinguished between the groups as children, now showed much greater overlap between them. Regression analysis indicated that although early language ability appeared to be related to outcome in the Autism group, in the Language group there was little association between measures of childhood functioning and later progress. The implications of these findings for understanding the nature of the underlying deficit in autism and the relationship between the two disorders are discussed.