Bilinguals sometimes report on difficulties in finding words while speaking in the first language (L1) or the second language (L2), which is frequently attributed to the negative influence of one language onto the other. This paper addresses this issue by investigating the relationship between L1 and L2 vocabulary knowledge of Korean-English bilinguals who have been exposed to an L2 learning environment since the age of 12 years. Their vocabulary knowledge in the L1 and L2 is measured by using a standardised vocabulary test in each language and their L1 vocabulary measures are compared with that of Korean monolingual 12-year-olds. The findings show that there is a significant positive correlation between the scores from the L1 and L2 vocabulary tests among the participants. The findings suggest (1) that L1 vocabulary learning continues in the L2 learning environment, and (2) that the extent of the vocabulary knowledge in one language is the good predictor of the other, highlighting the positive role of L1 vocabulary knowledge in L2 learning. Implications for the support for L1 development of young immigrants are also discussed. (Contains 3 tables and 4 notes.)