Background: Different approaches to measuring physical activity and fatness in youth have resulted in studies reporting relationships ranging from very strong to nonexistent. Methods: The sample comprised 787 boys and 752 girls between the ages of 7 and 16 years. Pedometer-determined physical activity, height, weight, and waist girth measures were taken. Results: Significant differences were found in activity level between body mass index–determined weight categories for the girls (F1,742 = 9.07, P = .003) but not for the boys (F1,777 = 3.59, P = .06) and between truncal adiposity groupings for the boys (F1,777 = 4.69, P = .03) and the girls (F1,742 = 13.56, P = .000). Conclusions: The relationship between physical activity and body fatness differs according to the measure used and between boys and girls. Factors contributing to body fatness such as eating behaviors or sedentary activities might be more important among boys than girls.