One of the most critical public concerns in the United States is the rapid increase in childhood obesity, partly due to the social and environmental changes (e.g., excessive TV and computer use, pressures of standardized testing, etc.) in the past few decades, which has resulted in less physical activity in school children's daily routines. Empirical studies have consistently shown a graded and negative relationship between BMI and physical activity among youth and adults. Increasing individuals' physical activity levels is associated with preventing or attenuating weight gain over time. Because no studies have been conducted in the United States to analyze the relationship between BMI levels and accelerometer-based physical activity among children and adolescents in physical education, the authors examined the effect of BMI on students' physical activity level in a middle school physical education class. They proposed three research questions: (a) what percentages of students are overweight and obese based on the BMI; (b) what is students' physical activity level in physical education as measured by accelerometers; and (c) are there significant differences in students' physical activity levels across different BMI groups (normal weight vs. overweight and obese). (Contains 3 tables.)