The present study investigated the relationship between lexicon and grammar in individuals who use graphic symbol-based aided augmentative and alternative communication (AAC). Data came from 60 transcripts of generalization sessions that were part of two previous intervention studies, aimed at improving the expressive vocabulary and grammar of 12 children and youth who used graphic symbol-based AAC. The specific aims of the current study were to (a) describe vocabulary composition across different levels of expressive vocabulary and (b) analyze the relationship between global measures of expressive vocabulary and the use of grammar in individuals who use aided AAC. A series of multiple linear mixed effect regression analyses showed a positive predictive association between overall vocabulary size and the use of closed-class words, and a positive relationship between the use of verbs and the use of closed-class words. Additionally, the use of verbs had a significant positive association with the use of inflectional morphology, while the use of nouns did not. Theoretical and practical implications of these findings are discussed.