This study examined effects of a school-based intervention called PLAY (Promoting Lifestyle Activity for Youth) on physical activity levels and body mass index (BMI) of students. Participants included 606 fourth-grade students selected from a stratified sample of 35 schools in Arizona and placed into four groups: PLAY & PE, PLAY Only, PE Only, and No Treatment. A treatment-control, post-measurement design assessed physical activity using the YAMAX pedometer, and height and weight data were collected following the intervention. One-way ANOVAs were used to analyze the data. Results indicated the treatment was effective at increasing the physical activity level of children, especially girls. No significant differences between groups were found for BMI. Programs such as PLAY, which can increase the activity level of children, may have significant health implications for youth.