A much under-researched issue in higher education is the extent to which English-medium university courses help students improve their English proficiency in an ESL context. Adopting a longitudinal, mixed methods design in which quantitative and qualitative data were collected and analyzed, the current study tracked English language improvement, or lack thereof, among 33 students over a 12-month segment of a Bachelor of Education degree (B.Ed.) program. The results revealed that on average, student scores showed very slight improvement both overall and across each of the four skill areas, although there was considerable variation among the students in the amount of improvement or lack of improvement made. Analysis of the interview data identified a combination of factors that might influence students' English language development in the B.Ed. program. Important implications of the results for pedagogical practices that facilitate ESL teacher trainees' English language proficiency development are discussed. Although this study took place in Hong Kong, it has value in other contexts where second or foreign language learners study disciplinary content through the target language.