The expressive lexical skills of 53 Polish bilinguals aged 24–36 months residing in the UK and Ireland were evaluated employing Polish and British English adaptations of the MacArthur-Bates Communicative Development Inventories. Polish vocabulary scores were collated to those of 53 Polish monolinguals matched for age, gender and parental education. The bilinguals were born to two Polish parents and mostly lived outside Poland since birth. Results displayed substantial differences in Total Conceptual Vocabulary and single-language vocabulary scores between the groups. However, the groups did not differ on Total Vocabulary measures. In the bilingual sample, there were significant correlations between children’s frequency of language use and their vocabulary scores in the same language. A negative correlation between children’s frequency of English use and their Polish vocabulary scores was found. An intricate pattern of factors relating to children’s low performance in Polish emerged. Hierarchical multiple regression analyses displayed that low birth weight, parental concern about language, maternal educational level as well as maternal frequency of Polish and English use contributed to clarifying children’s Polish vocabulary scores. Overall, results pointed out the need for early additional support of the first language (L1) if long-term balanced bilingualism is to be obtained.