This study compared the effect of physical activity and obesity on academic achievement and was based on the premise that the health of a child has an effect on his or her ability to learn and to achieve academically. Specifically, health-related topics of inactivity and obesity were considered. The participants included 12,607 third grade children entering kindergarten for the first time during the 1998-99 school year. The data were obtained from the National Center for Educational Statistics' Early Childhood Longitudinal Study (Third Grade), which is a national representative sample of students entering Kindergarten in 1998-99 with the latest wave of individual student, parent, teacher, administrator and school data collected on these same students in third grade. Third grade is a crucial year in elementary school as high stakes exams begin in third grade in most states. The results indicated that the Body Mass Index (BMI) of students, as well as the opportunity for physical activity within the school day affected the students' performance in both reading and mathematics achievement. The implications for school and district leaders are discussed with direction for future research presented.