This study describes the relationships between sedentary activity and body composition in 1,458 sixth-grade girls from 36 middle schools across the United States. Multivariate associations between sedentary activity and body composition were examined with regression analyses using general linear mixed models. Mean age, body mass index, and percentage of body fat were 12.0 +/- 0.51, 21.1 kg/m2 +/- 4.8, 28.5 +/- 8.9, respectively. Girls averaged 7.7 +/- 1.2 sedentary hours per day and about 13 hr (approximately 97% of the day) of both sedentary and light activities. Overweight girls were significantly more (13 min; p < .003) sedentary, especially after school (> 2 p.m.; p < .01), and less physically active (p < .0001) than normal weight girls. The study documents small but significant associations between sedentary activity and adiposity.