The Vocabulary Size Test (VST) measures English learners' decontextualised receptive vocabulary knowledge of written English and has nine bilingual versions with multiple-choice options written in other languages. This study used the English-Japanese version of the VST to investigate the extent to which loanword items were answered correctly by Japanese first language (L1) university students compared to non-loanword items, and whether it was easier to answer these loanword items when the correct answer option was written in loanwords rather than Japanese-words. Paired t-tests showed a significant difference in correct response rates between the loanword and non-loanword items, and the loanword options and Japanese-word options, with a large effect size. The results suggest the relative ease of learning English loanwords compared to non-loanwords for L1 Japanese users, and the need to consider the use of loanwords in vocabulary tests to measure test-takers' vocabulary size more accurately.