There is a common expectation, particularly amongst international students, that studying in an English-medium university should automatically produce a significant improvement in their English language skills. However, there is growing evidence to suggest that this is not necessarily the case. This paper reports on a study which investigated the impact of one semester of study at a university on the English language proficiency of a sample of 40 international students. This was measured by comparing the students' scores on a diagnostic English language test at the beginning and end of their first semester. A comparison of discourse measures of writing in terms of fluency, complexity and accuracy was also undertaken. Background information, including details of ESL support, if any, was collected for all participants via questionnaires, and interviews were conducted with a subset of the participants. It was found that studying in an English-medium university generally led to an improvement in English language proficiency. The paper identifies a number of factors which appear to support language development, as well as factors that may inhibit it. (Contains 3 tables, 1 figure, and 4 endnotes.)