The aim of the study was to determine the effectiveness of a school-initiated cognitive and behavioral program to reduce childhood obesity. Height, weight, body mass index (BMI), and BMI z scores were obtained at the beginning and end of the school year at an intervention school (n = 1022) and at a control school (n = 692). The prevalence of overweight and obesity was 18.9% and 30.4% versus 19% and 30.2%, respectively, in the intervention and control schools. The incidence of overweight increased in the control school, but the incidence of obesity, weight, and BMI z scores increased significantly in the intervention school, suggesting that implementation of any school-based obesity intervention programs requires careful planning to achieve goals.