Objective: To determine whether increased outdoor play time at Head Start was associated with greater changes in body mass index (BMI) over the course of a preschool year. Method: The authors used data from 2810 children from the Family and Child Experiences Survey 2006 cohort. With children's spring BMI as the outcome (both continuously measured and dichotomized to measure the risk of obesity), the authors conducted weighted regression analyses, controlling for child-level, family-level, and school-level covariates, including preschool entry BMI. Results: Children played outdoors at school for roughly 37 minutes per day, with little variation across half-day and full-day programs. The more children played outdoors, the more their BMI decreased over the preschool year ([beta] =-.05, 95% confidence interval (CI) [-0.08 to -0.01]) and the less likely they were to be obese (odds ratio = 0.99, 95% CI [0.98-0.99]). The difference between high levels and low levels of outdoor play corresponded to 0.18 BMI points and a 42% reduction in children's risk of obesity. Sixty minutes was the "tipping point" for the association between outdoor play time and improvements in children's BMI. These associations were also stronger among children who were obese at the start of the year, less active at home, and living in unsafe neighborhoods. Conclusion: Outdoor play time at Head Start is associated with decreases in children's BMI scores and, thus, may serve as an important means of preventing obesity. Head Start programs should consider establishing clear guidelines encouraging more outdoor time. (author abstract)