Introduction
The prevalence of overweight and obesity among American Indian youth may be 2 to 3 times higher than the national average. Whether weight gain during discrete out-of-school periods is occurring and contributing to the prevalence of overweight and obesity in this population is unknown.

Methods
We obtained repeated cross-sectional body mass index (BMI) samples from third-, fourth-, fifth-, seventh-, and eighth-grade boys and girls who reside on the Wind River Indian Reservation in central Wyoming. We collected measures at the beginning of 2 school years (N = 251), during 2 holiday breaks (N = 226), and during 1 summer recess (N = 141). We determined prevalence of normal weight and overweight among participants by grade level, and we calculated paired comparisons of BMI, BMI z score, and weight status during the holiday breaks and summer recess.

Results
Combined prevalence of at risk for overweight and overweight was 62.0% for boys and 56.6% for girls. For fifth-grade girls, significant increases in BMI ( P = .01) and z score ( P < .001) occurred over the holiday break. BMI increased significantly over the summer among third- and fifth-grade girls and among fourth-grade boys, but changes in z scores were nonsignificant. We observed an increase in weight status by out-of-school time in BMI ( P < .001) for schoolchildren at or above the 85th BMI percentile over the summer recess, but corresponding z scores did not change.

Conclusion
Prevalence of overweight among American Indian schoolchildren was higher than national estimates and higher than the prevalence in other similarly aged American Indian youth. Increases in BMI during out-of-school periods are likely due to normal growth, except among fifth-grade girls.