Background: In response to the Healthy, Hunger-Free Kids Act of 2010, the US Department Agriculture updated the nutrition standards for school meals to improve nutrition-related health outcomes for children. This study assesses the association between implementation of the updated nutrition standards and child weight. Methods: A quasi-experimental design and multivariate regression analysis is used to compare longitudinal changes in BMI z-scores for school lunch participants and nonparticipants across 2 nationally representative third grade cohorts of children--those exposed to the updated standards (N = 5480) and those not exposed (N = 9240). Results: Boys who ate school lunches after implementation of the updated standards experienced slower BMI z-score growth than did nonparticipants; participants' BMI z-scores increased by 0.10 compared with 0.18 for nonparticipants. We find no relationship between school lunch participation and BMI z-scores for girls. Conclusions: This study provides evidence that the updated nutrition standards for school meals are associated with improved weight outcome for boys, but have no effect for girls, 1 year after implementation. These findings are encouraging and provide support for continued focus on improving the nutritional quality of school meals.