Aims: The purpose of this study was to determine the association between amount of physical activity and body mass index (BMI) percentile among middle and high school aged children. Background: The epidemic of physical inactivity among youth has substantially contributed to the obesity epidemic. Both in-school and out-of-school environments need be considered when evaluating physical activity among children. Data source: Data were drawn from the Panel Study of Income Dynamics (PSID) Child Development Supplement, 2007. Methods: A cross-sectional design was employed to examine a sample of 1,306 children. The dependent variable was BMI percentile, while the independent variable was physical activity. Multinomial logistic regression model was used to assess the associations between physical activity and BMI percentile controlling for age, gender, race/ethnicity, parental income, participation in school lunch programs, and neighborhood safety. Results: After adjusting for covariates, children who engaged in low daily physical activity levels had 1.8 times the odds of being obese vs. normal weight than those who engaged in moderate levels (OR = 1.80, CI = 1.31, 2.48). Minority children and females exhibited higher odds of being obese in comparison to non-Hispanic white children. Conclusions: This study suggests that adherence to national physical activity guidelines in- and out-of-school may be effective in preventing obesity among children.