Research has shown that children repeat high-probability phoneme sequences more accurately than low-probability ones. This effect attenuates with age, and its decrease is predicted by developmental changes in the size of the lexicon (J. Edwards, M. E. Beckman, & B. Munson, 2004; B. Munson, 2001; B. Munson, J. Edwards, & M. Beckman, 2005). This study expands on these findings by examining relationships between vocabulary size and repetition accuracy of nonwords varying in phonotactic probability by 16 children with specific language impairment (SLI), 16 chronological-age-matched (CA) peers with typical speech and language development, and 16 younger children matched with the children with SLI on vocabulary size (VS). As in previous research, children with SLI repeated nonwords less accurately than did CA children. The children with SLI and the VS children showed similar levels of nonword repetition accuracy. Phonotactic probability affected repetition accuracy more for children with SLI and VS children than for CA children. Regression analyses showed that measures of vocabulary size were the best predictor of the difference in repetition accuracy between high- and low-probability sequences. Analyses by items showed that measures of phonotactic probability were stronger predictors of repetition accuracy than judgments of wordlikeness. Taken together, the results support research demonstrating that vocabulary size mediates the influence of phonotactic probability on nonword repetition, perhaps due to its influence on the ongoing refinement of phonological categories. (Contains 3 figures, 1 footnote, and 5 tables.)