Several studies provide considerable insight into the role that frequency plays in language development. However, no study has investigated the direct relationship between frequency and grammatical acquisition in children with specific language impairment (SLI). In this study, we focus specifically on the influence of the frequency of functional words on the ability of children with SLI to produce grammatical constructions based on the children's own previous production but containing previously unused functional words. To test our hypothesis, the children were administered an imitative repetition task, tailored to their current level of language development. Results showed that children with SLI performed more poorly than language-matched children with typical language development. The difference between the two groups was more marked when the previously unused functional words were infrequent rather than frequent. Consequently, it would seem that the token frequency of functional words influences grammatical acquisition in children with SLI. The results and their implications for linguistic theories are discussed.