Background: Over the years schools have reduced physical education and recess time in favor of more academic instruction. Due to the drastic rise in obesity levels among children, some states have begun to mandate minimum amounts of physical activity (PA) that school children receive, causing schools to find alternative methods for increasing PA levels. Physical Activity Across the Curriculum (PAAC) is a 3-year randomized clinical trial incorporating moderate-intensity PA in elementary schools to reduce childhood obesity. This paper describes the rational, design, and methods of the PAAC intervention study. Methods: Twenty-two elementary schools were randomized to either a control or intervention condition. In schools randomized to the intervention condition (PAAC), regular classroom teachers were taught how to incorporate PA into standard academic lessons. Teachers were asked to accumulate 90-100 min/week of PAAC each week through out the 3-year study period. Schools randomized to the control group did not alter their teaching methods. Direct observation of PA levels in the classroom was collected weekly. Height and weight was measured twice a year to calculate BMI. Results: Two years of the intervention have been completed and only one school has left the study. The remaining 21 schools are participating in the final intervention year. Conclusions: The results from the PAAC intervention may provide schools with an alternative method to increase PA levels in children and reduce childhood obesity. © 2007 Elsevier Inc. All rights reserved.