The assertive speech acts of children with autism (n = 12) and Asperger syndrome (n = 12), individually matched to children with specific language impairment (SLI; n = 24) and children with normal development (n = 24) were studied in the context of gently structured conversation. These children also completed the false belief test of theory of mind. The children with autism used significantly lower proportions of assertions involving explanations and descriptions than the children with SLI or normal development and significantly lower proportions of assertions involving internal state and explanations than the children with Asperger syndrome. The children with autism used a higher proportion of assertions involving identifications than any other group. The assertions of the children with Asperger syndrome were generally not different than those of the children with SLI or normal development except for a higher proportion of assertions involving own internal state. Further analysis of the mental assertions revealed that the children with autism and Asperger syndrome predominantly referred to desire and made few references to thought and belief, whereas the children with SLI and those with normal development used a higher proportion of references to thought and belief.