About 10% of primary school students in developing countries have poor vision, but very few of them wear glasses. Almost no research examines the impact of poor vision on school performance, and simple OLS estimates are likely to be biased because studying harder often adversely affect one’s vision. This paper presents results from a randomized trial in Western China that offered free eyeglasses to 1,528 rural primary school students. The results indicate that wearing eyeglasses for one year increased average test scores of students with poor vision by 0.15 to 0.22 standard deviations, equivalent to the learning acquired from an additional 0.33-0.50 years of schooling, and that the benefits are greater for under-performing students. A simple cost-benefit analysis suggests very high economic returns to wearing eyeglasses, raising the question of why such investments are not made by most families. We find that girls are more likely to refuse free eyeglasses, and that lack of parental awareness of vision problems, mothers’ education, and economic factors (expenditures per capita and price) significantly affect whether children wear eyeglasses in the absence of the intervention.