Research suggests that people with intellectual impairments who use manual signs to augment or substitute for speech rarely progress beyond the stage of single signs and that word order is particularly problematic. However, the majority of studies have focused on experimental tasks, and relatively little is known about spontaneous sign production in naturalistic settings. The present study explored the linguistic development in sign and speech of 10 children who relied on manual signs (the Makaton vocabulary) as their main means of communication. Mean utterance length in sign ranged from 1.0 to 2.5, and analysis of semantic relations, lexical development, and word order suggested that the children had not developed their language beyond MLU Stage I. Examination of their abilities within the modality of sign indicated that some children were able to manipulate features of sign at a sublexical level. The results are discussed in relation to the language input by teachers, and inferences are drawn regarding the underlying modality of linguistic representation in children who use manual signs.