Interest in the benefits of exercise has prompted increased research examining the relationship between physical activity and health status in adults. More recently, considerable research effort has been directed toward the role of physical activity in young children as a precursor to adult physical fitness. The purpose of this study was to estimate the relationship between physical activity measured via accelerometry and body mass index, body mass, body composition, and physical fitness in fourth-grade boys and girls during physical education lessons. 54 fourth-grade students in rural Pennsylvania (28 girls, M age 9.6, SD = .56; 26 boys, M age 9.5, SD = .51) were subjects. Girls were significantly less active during all lessons measured (p < .001). The results of linear regression analyses indicated that physical activity was negatively associated with body mass index, body mass, and body fat percentage (p < .05), and positively associated with physical fitness (VO2max) (p < .01). These data indicate the need for curricular intervention to motivate girls to increase their activity during structured physical education lessons and demonstrate the efficacy of the body mass index as a screening tool within the schools.