Elevated body weight among active duty Air Force (ADAF) members is a substantial and growing problem, and typically results from gaining small amounts of weight each year over many years. We designed a strategy to prevent annual weight gain in ADAF members using self-directed behavior change booklets followed by weekly e-mails about diet and physical activity for a year. The intervention was universally offered to ADAF members meeting selection criteria at five U.S. Air Force bases (n = 3,502); members at 60 other U.S. Air Force bases served as controls (n = 65,089). The intervention was completely effective at preventing weight gain in a subgroup of men (those above the lowest three ranks, with baseline weight above maximum allowable) and in women, while controls continued to gain weight. Since the intervention did not require personalized contact, this approach has promise for large-scale population-based efforts aimed at preventing weight gain in working adults.