Sixty-one women with newly diagnosed locoregional breast cancer (T1-3, N0-1, M0) having an initial Body Mass Index (BMI) between 20 and 35 kg/m2 who were receiving standard adjuvant treatment (chemotherapy, tamoxifen, and/or radiation) were asked to avoid weight gain (if initial BMI < or = 25 kg/m2) or to lose up to 10 kg (if initial BMI 25-35 kg/m2) over one year. Women participated in twenty group sessions (10 weekly, 10 monthly) which involved a psychological supportive-expressive group intervention supplemented by individual weight goals, and nutrition and exercise programs. Fifty-five non-censored women (5 developed recurrence, 1 died of a subarachnoid hemorrhage) lost a mean of 0.53 +/- 3.72 kg. Weight loss was greatest in initially overweight women (BMI > or = 25 kg/m2) who lost 1.63 +/- 4.11 kg (p = 0.01 compared to normal weight women) and in those not receiving chemotherapy who lost 2.15 +/- 2.83 kg (p = 0.0004 compared to those receiving chemotherapy). 70.9% met predefined criteria for success. Aerobic exercise increased significantly during the intervention (p = 0.00005) and was the strongest predictor of success (OR 1.73 for each additional 30 minutes of exercise weekly, p = 0.003). Changes in caloric intake were not significant, but fat intake decreased and carbohydrate and fibre intake increased significantly during the intervention. Eating behavior and psychological status improved significantly. Thus, this multidisciplinary weight management intervention successfully prevented weight gain in women with newly diagnosed locoregional breast cancer, and helped overweight women lose weight.