Theory of mind (ToM) development by a sample of 63 children aged 5-12 years (24 with Asperger syndrome, 19 with high-functioning autism, and 20 age-matched typical developers) was assessed with a five-task false-belief battery in relation to both lexical (vocabulary) and syntactic (grammar) language skills. Contrary to some previous research, no differences in ToM emerged between those with Asperger syndrome and their typically developing peers but those with autism were delayed substantially behind both other groups in ToM understanding, even after controlling for age, non-verbal ability and verbal (both lexical and syntactic) mental age. For all the diagnostic groups equally, syntax was a more important ToM predictor than lexical language skill. Theoretical and practical implications of the findings are discussed. (Contains 2 tables.)