Objective: To examine the association between body mass index in young adolescents and schoolwide food practices such as foods used in school fundraising and in the classroom as incentives and rewards. Design: Using a cross-sectional study design, we collected data from both the schools and the students. School administrators provided information on schoolwide food policies and practices. Eighth-grade students provided self-reported heights and weights. Setting: Sixteen middle schools in the Minneapolis-St Paul metropolitan area. Participants: The study included 3088 eighth-grade students. Students were participants in a school-based dietary intervention study. Main Outcome Measures:: Body mass index was calculated from self-reported height and weight data. A 7-item schoolwide food practices scale (Cronbach α=0.83) was created using data collected from school administrators. Results: The mean number of food practices permitted by a school was 3 (range, 0-7). The most prevalent food practices were the use of food as incentives and rewards (69%) and in classroom fundraising (56%). Body mass index of the students increased 10% for every additional food practice permitted in their school (P<.03). Conclusions: Schoolwide food practices that supported frequent snacking and the consumption of foods and beverages high in calories and low in nutrients by students throughout the school day were common and adversely associated with body mass index of the students. Prevention of overweight in childhood must include attention to the nutrition integrity of schools, and school nutrition policies that consistently support and promote healthy dietary practices among young adolescents are urgently needed.