This paper reports on an investigation of Turkish English-medium university students' perceptions of the change in their language ability and difference between students' level of English proficiency when they start their departmental studies and in their 4th semester and onwards based on their proficiency exam scores. Results revealed that 1) studying in an English-medium university improves students' reading, listening and overall English proficiency significantly 2) there is not a statistically significant change between student's pre and post writing scores 3) the empirical findings are line with participants' perceptions of the impact of studying in an English-medium university on their language development. These findings suggest insightful implications for English-medium universities in Turkey and all around the world.