A sample of 37 children aged 4-7 years who all showed some autistic features was investigated. Children with a primary diagnosis of autism were compared with those diagnosed with a language disorder, on behaviours within four domains; social behaviour, imaginative activities, repetitive behaviour and communication. The aim was to identify potentially differentiating features of the two groups using observational ratings and questionnaire measures provided by parents and teachers. Information on participants' intelligence and language skills was also collected. The children with autism showed greater deficits in joint attention, functional play and pragmatic language, and engaged in more repetitive behaviours, than the language disordered children. However, the groups did not differ significantly on formally assessed language skills. A cluster analysis produced three groups of children varying in level of functioning and parent-rated behaviours. The results are informative for clinicians dealing with the challenge of differential diagnosis.