Background: Research shows that elementary students gain weight over the summer. It is unknown if these findings apply to Hispanic adolescents. We evaluated school and summer standardized body mass index (zBMI) changes in Hispanic middle school students. Methods: Hispanic middle school students were randomly assigned to a weight management program (N = 230) or control condition (N = 195). Paired sample t-tests compared zBMI change scores during the school year versus summer when not enrolled in a weight management program (eg, control group). We used 2 × 2 repeated measures analysis of variances to determine program group differences in zBMI scores across school versus summer periods separately for normal and overweight/obese students. Results: In the control group, students' zBMI change scores decreased during the school year, but increased during summer for both normal weight and overweight/obese students. Program effects on zBMI indicated school year decreases and summer increases for both normal and overweight/obese students. However, zBMI scores did not differ by program status for normal weight students. Overweight/obese students in the program compared to the control group increased zBMI to a lesser extent over the summer. Conclusion: For all students, zBMI increased during the summer. A school-based weight management program protected overweight/obese students against potentially greater summer weight gain.