The full-scale trial started in September 1996. The primary aim was to test the effectivenes of a culturally appropriate school-based intervention that promoted increased physical activity and healthful eating behaviors to prevent obesity in American Indian upper elementary school children. The primary outcome was percent body fat after three years of intervention. The major hypothesis was that intervention would result in an absolute difference of 3 percent body fat between the control and intervention schools. All students enrolled in the second grade in the participating schools during Spring, 1997 participated in the baseline survey. Baseline measurements were performed at the end of second grade so that the intervention could begin at the start of third grade. The intervention lasted for three years. A single cohort of children progressed through third, fourth, and fifth grades. Schools were the unit of randomization. Forty-one schools in four centers were stratified within each field center on median percent body fat (PBF) at the end of second grade using data from the baseline measurement. Schools within a field center were then ranked on median PBF. Once stratified, the coordinating center randomly assigned half the schools within each stratum to intervention (21) and half to control (20).