To facilitate second language learning, it has become increasingly popular to use a second language as the medium of instruction for content subjects for majority language students. Although numerous research studies have shown the advantages of such kind of programs in North America and Europe, those investigating English as the Medium of Instruction (EMI) schools in Hong Kong yielded inconclusive results. This meta-analysis is the first attempt to synthesize the research evidence on EMI education in Hong Kong since 1970. Based on 24 studies, this meta-analysis shows that students in EMI secondary schools were more proficient in second language and performed better on measures of affective variables. Yet their learning in other content subjects suffered. The differences between the effectiveness of EMI education in Hong Kong and that of similar programs in other contexts will be discussed, thereby illuminating second language acquisition theories and bilingual education.