Accurate information about weight loss is not available for most commercial weight-loss programs. Our objective was to obtain accurate assessments of weight outcomes, behavioral data, and side effects for an intensive behavioral weight-loss program using low-energy diets. Weights, behavioral data, and side effects for 173 consecutive patients were assessed. Treatment options were Medically Supervised, using a minimum of five meal replacements per day, and Healthy Solutions, using meal replacements, fruits, and vegetables. Both options included weekly classes, daily records, midweek phone calls, and a minimum physical activity goal of 2,000 kcal/week. The primary outcome was change in body weight with secondary outcomes related to behavioral changes and side effects. Intention-to-treat (ITT) and completer analyses (completed 9 weeks of treatment) were performed. During 13 months, 173 patients met criteria for analysis. They selected either Healthy Solutions: 56 patients (ITT) and 37 completers; or Medically Supervised: 117 patients (ITT) and 93 completers. Mean (+/-standard error) weight losses were: Healthy Solutions: ITT=12.8+/-1.3 kg in 13 weeks and completers=17.0+/-1.4 kg in 18 weeks; Medically Supervised: ITT=16.6+/-1.0 kg in 16 weeks and completers=19.7+/-1.0 kg in 19 weeks. Mean use of meal replacements, fruits, and vegetables exceeded goals. Most side effects were mild and did not affect program continuation. An intensive behavioral weight-loss program using low-energy diets, including five meal replacements daily, is safe and effective.