PURPOSE: This study aimed to identify predictors of changes in weight and body composition among women receiving chemotherapy for breast cancer. PATIENTS AND METHODS: Data were from 49 women age 40-54 receiving chemotherapy for breast cancer. Weight, height, and body composition measurements from dual-energy x-ray absorptiometry (DEXA) scanning were completed at baseline (within 1 month of beginning chemotherapy) and 12 months. Caloric intake was assessed from food diaries at baseline, 6 and 12 months, and physical activity was measured by questionnaire at baseline, 3, 6, 9, and 12 months. RESULTS: Baseline body mass index (BMI) was inversely associated with gains in weight (P = .01) and fat mass in torso (P = .006). Women of normal weight gained an average of 4.3 pounds and increased fat mass in torso and arms. Overweight women lost 3.0 pounds, and obese women lost 4.1 pounds, and neither group increased body fat. Decreased physical activity was associated with weight gain (P = .047). Additional predictors of increased fat mass in torso were younger age (P = .023) and treatment with tamoxifen (P = .015). Predictors of loss of bone mineral content included older age (P = .004) and treatment with aromatase inhibitor (P = .024), whereas treatment with bisphosphonate prevented bone loss (P < .0001). CONCLUSION: Women receiving chemotherapy for breast cancer who are of normal weight at the time of breast cancer diagnosis are more likely to gain weight and body fat during the following year than overweight or obese women.