The ability to talk about the internal states (IS) of self and other is an age-typicaldevelopment of early childhood that is thought to reflect young children's emergentself–other understanding. This study examined the emergence of an IS lexicon in across-sectional sample of young children with Down syndrome (DS) and a cognitively anddemographically comparable group of normally developing (ND) children. Children's ISlexicons were derived from transcripts of their spontaneous utterances during two laboratorycontexts: a mother–child emotions picture book task and semistructured play. Childrenwith DS produced significantly fewer IS words and fewer IS word types than their MA-matchedcounterparts. Controlling for corpus size, children with DS also were less likely to attributeinternal states to themselves and were more context bound in their use of IS language. Inaddition, children with DS also differed from ND children in the semantic content of their ISlanguage, with proportionately higher rates of affective words and lower rates for words aboutvolition, ability, and cognition. For both the DS and ND groups, individual differences in ISlanguage production were significantly related to general expressive language skills. However,dissociations were observed for the relation between children's IS word production andnonverbal symbolic play skills in the two groups. These findings suggest some degree ofdisorganization at the interface among symbolic domains for children with DS. Because ISlanguage is critical to the regulation of social interaction and an early index of self–otherdifferentiation and understanding, children with DS may be at risk for later compromises inself-organization.