Sixteen children with language delays and their mothers were studied to identify the types of child utterances mothers were most likely to expand. Eight of these children had Down's syndrome (DS), while the other eight were pairwise-matched for mean length of utterance (MLU) and did not have DS, but were language delayed. Twenty-minute mother-child free-play sessions were videotaped and transcribed. Trained observers coded utterances for child intelligibility, child utterance length, adult expansions and adult non-expansions. Sequential analysis results indicated that mothers of children with DS were more likely to expand partially intelligible multi-word utterances than to expand fully intelligible multi-word utterances. The opposite pattern occurred in the dyads without DS. Single-word utterances were least likely to be expanded in both groups. The implications of the results for language intervention and future research are discussed.