PURPOSE: Weight gain is a common problem among breast cancer patients who receive adjuvant chemotherapy (CT). We undertook a study to determine the causes of this energy imbalance. PATIENTS AND METHODS: Factors related to energy balance were assessed at baseline (within 3 weeks of diagnosis) and throughout 1 year postdiagnosis among 53 premenopausal women with operable breast carcinoma. Thirty-six patients received CT and 17 received only localized treatment (LT). Measures included body composition (dual energy x-ray absorptiometry), resting energy expenditure (REE; indirect calorimetry), dietary intake (2-day dietary recalls and food frequency questionnaires) and physical activity (physical activity records). RESULTS: Mean weight gain in the LT patients was 1.0 kg versus 2.1 kg in the CT group (P =.02). No significant differences between groups in trend over time were observed for REE and energy intake; however, a significant difference was noted for physical activity (P =.01). Several differences between groups in 1-year change scores were detected. The mean change (+/- SE) in LT versus CT groups and P values for uncontrolled/controlled (age, race, radiation therapy, baseline body mass index, and end point under consideration) analysis are as follows: percentage of body fat (-0.1 +/- 0.4 v +2.2 +/- 0.6%; P =.001/0.04); fat mass (+0.1 +/- 0.3 v +2.3 +/- 0.7 kg; P =.002/0.04); lean body mass (+0.8 +/- 0.2 v -0.4 +/- 0.3 kg; P =.02/0.30); and leg lean mass (+0.5 +/- 0.1 v -0.2 +/- 0.1 kg; P =.01/0.11). CONCLUSION: These data do not support overeating as a cause of weight gain among breast cancer patients who receive CT. The data suggest, however, that CT-induced weight gain is distinctive and indicative of sarcopenic obesity (weight gain in the presence of lean tissue loss or absence of lean tissue gain). The development of sarcopenic obesity with evidence of reduced physical activity supports the need for interventions focused on exercise, especially resistance training in the lower body, to prevent weight gain.