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 [plus or minus] 0.51, 21.1 kg/m[superscript 2] plus or minus 4.8, 28.5 [plus or minus] 8.9, respectively. Girls averaged 7.7 plus or minus 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 less than 0.003) sedentary, especially after school (greater than 2 p.m.; p less than 0.01), and less physically active (p less than 0.0001) than normal weight girls. The study documents small but significant associations between sedentary activity and adiposity. (Contains 5 tables and 1 figure.)