Background: This study evaluated a school-based obesity intervention for elementary school children (N = 835) where health professionals assisted teachers with the integration of healthy messages into the school curriculum. Methods: Schools were randomized into a professional-facilitated intervention (PFI; N = 4) or a self-help (SH; N = 3) condition. Changes in weight-based outcomes were assessed in students enrolled in the second grade from all 7 schools (overall: N = 835 students; PFI: N = 509 students, SH: N = 326 students). Students were between ages 7 and 9 and from diverse ethnic backgrounds (Asian = 25.3%, Black = 23.3%, Hispanic = 23.1%, White = 28.3%). The sample included 321 overweight/obese (BMI greater than or equal to 85th percentile), 477 normal-weight (BMI greater than or equal to 5th percentile and less than 85th percentile), and 37 underweight (BMI less than 5th percentile) students. Results: After 2 years, children who were overweight/obese in the PFI condition signi?cantly reduced their standardized BMI (zBMI) compared to children in the SH condition (Wald [chi [superscript 2] = 28.7, p less than 0.001). End-of-year grades decreased for overweight/obese students in both conditions; however, students in the PFI exhibited a smaller decrease in grades compared to the SH condition (Wald chi[superscript 2] = 80.3, p less than 0.001). Conclusion: The results indicate that an obesity prevention program where health professionals assist teachers by integrating healthy messages into existing curriculum was effective in reducing zBMI compared to the SH condition. (Contains 4 tables and 3 figures.)