Many children with communication impairments whose educational needs cannot be met in mainstream classrooms receive additional resource support in classroom units that specialise in the coordination and provision of academic teaching and speech language therapy in the UK. This study estimated the prevalence of Specific Language Impairment (SLI) among the children attending these classroom units. In nine classroom units with a total of 147 pupils, the prevalence of SLI was estimated to be 13%. The receptive-expressive SLI group comprised 10% of the sample, and the expressive-only group comprised 3%. An additional borderline language-impaired group was identified by only one of the four language measures used, that is, sentence repetition. It is argued that the abilities of specialised classroom units to meet the educational needs of children with SLI and other communication impairments may be challenged by an increasingly diverse intake.