This paper discusses the impact of early elementary school attendance on children’s body weight. Using data from the Early Childhood Longitudinal Study–Kindergarten Cohort of 1998 (ECLS-K), researchers compared the weights of children who had completed first grade to those of the same age who had completed kindergarten only. Employing a regression-discontinuity (RD) design, researchers found no strong evidence that an additional year of school had either positive or negative effects on weight outcomes in the full sample. However, additional school exposure appeared to improve weight outcomes of children for whom the transition to elementary school represented a more dramatic change in environment, such as those who spent little time in child care prior to entering kindergarten.