The present study examines the language use of 1,333 students taking English medium instruction (EMI) courses in four elite universities in South Korea. It employs a quantitative method with qualitative comments. The study investigates three language practice issues in EMI courses, including: (i) the extent of use of English in EMI courses, (ii) students' use of English outside the university and (iii) students' language mixing practices. The results show that the reality of EMI classes encompasses the parallel use of both Korean and English. This study also finds that while Korean is the dominant language outside the academic context, there is increasing use of English by students in social settings. This study argues that acknowledging the mixed-use of languages in EMI classes is critical and suggests implementing a dual language education policy. Finally, it discusses the impact of EMI on the language behaviour of Korean students and challenges a long-held view of a wholly monolingual Korean society.