In dialogue, speakers tend to imitate, or align with, a partner's language choices. Higher levels of alignment facilitate communication and can be elicited by affiliation goals. Since autistic children have interaction and communication impairments, we investigated whether a failure to display affiliative language imitation contributes to their conversational difficulties. We measured autistic children's lexical alignment with a partner, following an ostracism manipulation which induces affiliative motivation in typical adults and children. While autistic children demonstrated lexical alignment, we observed no affiliative influence on ostracised children's tendency to align, relative to controls. Our results suggest that increased language imitation--a potentially valuable form of social adaptation--is unavailable to autistic children, which may reflect their impaired affective understanding.