Technical Abstract: Most weight-loss research targets obese individuals who desire large weight reductions. However, evaluation of weight-gain prevention in overweight individuals is also critical as most Americans become obese as a result of a gradual gain of 1-2 pounds per year over many years. This study evaluated the efficacy of an Internet-based program for weight loss and weight-gain prevention with a two-group, prospective, randomized controlled trial. A military medical research center with a population of 17,000 active-duty military personnel supplied 446 overweight individuals (222 men; 224 women) with a mean age of 34 years and a mean BMI of 29. Recruitment and study participation occurred 2003-2005, and data were analyzed in 2006. Participants were randomly assigned to receive the 6-month behavioral Internet treatment (BIT, n=227) or usual care (n=224). Change in body weight, BMI, percent body fat, and waist circumference, presented as group by time interactions, were measured. After 6 months, completers who received BIT lost 1.3 kg while those assigned to usual care gained 0.6 kg (F((df=366))=24.17; I<0.001). Results were similar for the intention-to-treat model. BIT participants also had significant changes in BMI (-0.5 vs +0.2 kg/m(2); F((df=366))=24.58); percent body fat (-0.4 vs +0.6%; F((df=366))=10.45); and waist circumference (-2.1 vs -0.4 cm; F((df=366))=17.09); p<0.001 for all. Internet-based weight-management interventions result in small amounts of weight loss, prevent weight gain, and have potential for widespread dissemination as a population health approach.