Built environment characteristics that may increase walkability generally were associated with lower BMI z-scores in a large sample of children and adolescents from an electronic health record database, especially perhaps among children in lower-income neighborhoods. Thus, this study, using large electronic health record data, suggests that neighborhood physical features can affect BMI z-scores. Our findings suggest that modifying existing built environments to make them more walkable may reduce childhood obesity. Public policies at local and national levels likely should promote walkable built environments.