The aim of this study is to investigate the effects of instruction in students' non-native language on the academic success of university students. To analyze this effect, we utilized the data from a Turkish university with many departments that offer the same bachelor degree programs both in English and in Turkish. All other aspects of the programs were assumed to be identical and students had very similar academic and socioeconomic backgrounds in both degree programs. Our analysis indicated that instruction in the non-native language affects negatively the academic success (i.e., semester point average) of students. This negative influence was higher in the first year but still existed by the fourth year. However, we found the opposite effect for the students with merit-based scholarships, who are arguably brighter and self-motivated. We should note that we were not measuring the benefits of English-medium instruction. Thus our results do not automatically imply that English-medium instruction is harmful to the students.