The utility of homework and its impact on academic achievement has been an ongoing debate for more than 100 years. To date, there is no rigorous experimental research to show whether, and the extent to which, variations in the amount of homework assigned can impact the academic performance of elementary school students. In this study, 440 second-grade students were randomly distributed in 3 groups within the classes they attended. Each group received different amounts of homework in writing and math for 20 days. The results showed a significant immediate effect of homework quantity on writing competency (but not on math competency). The writing homework effects were sustained 4 months later, but only for the group that had allocated a moderate amount of homework to writing skills practice. This study shows that the additional opportunities for practice offered by homework can have differential short-term and medium-term effects on the academic achievement of elementary school students and supports the assignment of a moderate amount of writing homework at this age.