Following the hand-over of Hong Kong to China in 1997, the medium of instruction in most secondary schools in Hong Kong was changed from English to Chinese. Concerns have been expressed that the English language proficiency level of pupils whose secondary school medium of instruction (MOI) is Chinese is lower than that of those whose MOI is English, and that they are thus disadvantaged when entering Hong Kong's English-medium universities. This study examines the effect that the change in policy has had on the size of students' English academic vocabulary, both passive and active, and the consequences of this. Two vocabulary tests were administered to obtain data from first-year students (n = 762) from eight faculties of one Hong Kong university. Subsequently, 413 of these subjects wrote an essay which was first processed using an on-line Vocabprofiler to identify lexical richness, and then assessed by an experienced language instructor to examine lexical appropriacy. The data derived from the tests were examined and also compared to those obtained in an earlier Hong Kong study (Fan, 2001). The results of the present study suggest that the change in language policy has had a significant negative effect on the size of academic vocabulary of Hong Kong tertiary students. (Contains 6 tables.)