English-only college education in non-English speaking countries is a rapidly growing phenomenon that has been dubbed as the most important trend in higher education internationalization. Despite worldwide popularity, there is little empirical evidence about how the transition to English-only instruction affects students' academic outcomes. Using a natural experiment at a selective university in Central Asia and a difference-in-differences strategy, we estimate the causal effect of switching to English-only instruction on students' college outcomes. We find that the introduction of English-only instruction led to a decrease of GPAs and probability of graduation and an increase in the number of failed course credits. Although negative, the effects were short-lived. The difference-in-differences estimates and the examination of potential mechanisms suggest that at least in selective universities in non-English speaking countries, the switch to English-only instruction may affect college outcomes negatively at the time of transition but may not necessarily imply longer-run negative effects.