This field-based randomized trial examined the effect of assignment to and participation in summer school for two moderately at-risk samples of struggling readers. Application of multiple regression models to difference scores capturing the change in summer reading fluency revealed that kindergarten students randomly assigned to summer school outperformed their control group peers by 0.60 of a standard deviation in an intent-to-treat analysis. For the first grade sample, the intent-to-treat estimate was over three quarters of a standard deviation. The contrast in performance was greater when the comparison was focused more specifically on the change in literacy between treatment participants (i.e., randomly assigned students who actually attended summer school) and students randomly assigned to the control group and in analyses that explicitly adjusted for non-compliance with treatment assignment. These results support the experiential intuition of school district personnel regarding the benefits of summer school and suggest that targeted summer instruction can be a useful strategy to support student learning over the summer months. (Contains 2 figures and 3 tables.)