Within higher education, it is often believed that the adoption of English-medium instruction (EMI) will provide domestic students with the language skills that will enable them to be more competitive in their future professional pursuits. However, research into language learning and EMI at the university level is very scarce. This study examined essays by 195 undergraduate students (99 1st-year and 96 3rd-year learners) at a major university in Spain in order to investigate the effect of three years of instruction through the medium of English on their level of proficiency, essay quality and lexical diversity (as measured by the Oxford Placement test, the CEFR writing scale and three measures of lexical diversity). The results showed a significant improvement in the learners' second language (L2) proficiency and a significant but subtle increase in essay quality ratings but no significant increase in lexical diversity scores. In the same vein as the findings of some of the other studies we discuss, the present findings seem to suggest the need to rethink and re-evaluate the implementation of EMI in contexts like the one examined in this study.