Background: Evidence suggests that children gain more weight during the summer months compared with the school year. To examine the impact of the school and summer environment on children's weight further, we conducted a 5-year longitudinal study examining changes in standardized BMI (zBMI) of students entering kindergarten. Methods: Heights and weights were obtained at the beginning and end of each school year for 3,588 ethnically diverse (Caucasian: 27.2%, Black: 29.0%, Hispanic: 26.4%, and Asian 17.4%) students aged 5-7. Results: A significant difference in change in zBMI during the school and summer months was found (-0.52, 95% CI: -0.59 to -0.45, p less than 0.001; Wald x[superscript 2] = 171.89, p less than 0.001). Overall, children decreased BMI percentile during time spent in school by 1.5 percentile points and increased by 5.2 percentile points during summer months. Differences in the velocity of weight gain were found across weight classification categories with only overweight and obese children decreasing their zBMI during the school year. Conclusion: Time spent in school was shown to have a beneficial impact on students' weight, especially for students who were overweight or obese. However, these results are alarming because weight gain during elementary school occurs primarily during the relatively short span of summer break. (Contains 2 tables and 1 figure.)