Background: This research explored the relative impact of demographic, cognitive, behavioural, and psycholinguistic factors on vocabulary development in two-year-old children. Methods: Two hundred and thirty-two children (24-30 months) were tested on expressive and receptive vocabulary, cognitive development, word learning and working memory skills. Parents completed a British adaptation (Klee & Harrison, 2001) of the MacArthur-Bates Communicative Development Inventory (CDI; Fenson et al., 1993), a demographic questionnaire and a questionnaire regarding the child's social-emotional behaviour. Results: Several demographic, child and processing variables were significantly correlated with CDI (vocabulary) scores, but the only significant unique predictors of CDI scores were nonword repetition (NWR; R[superscript 2] change = 0.36), sex (R[superscript 2] change = 0.05) and age (R[superscript 2] change = 0.04). Scores were only included when a child completed the entire NWR test (77% of toddlers). Conclusions: The NWR task used in this experiment maximised participation in this group of toddlers, and was a strong predictor of vocabulary ability. Longitudinal research is warranted to explore the independent and reciprocal growth in working memory and language skills in children.