Background: Evidence has linked weight gain and obesity to increased breast cancer risk, poorer outcomes, decreased survivorship, and increased risk of recurrence. Adjuvant therapies can increase weight gain, making tailored exercise interventions beneficial in maintaining a healthy weight. In this study, body composition changes were examined after a 12-week exercise program in a cohort of breast cancer survivors. Methods: The Breast Cancer Exercise Rehabilitation Research Study recruited breast cancer survivors to undergo a 12-week exercise program (NCT04013568) and examine body composition changes. Measurements taken included anthropometry (waist circumference, waist-to-hip ratio, BMI), DXA body composition measurements (fat mass-visceral and subcutaneous, trunk to limb, muscle mass-appendicular, and whole body), and BIA (Bio-electrical Impedance Analysis-muscle, fat, and water). 34 participants completed the exercise program and baseline and post-12-week measurements were obtained. Descriptive statistics and a paired t-test analysis was done to analyze changes in body composition using SAS onDemand. A correlation analysis between measurements simple to do in clinic and DXA measurements were run to determine proxy use for DXA. Results: After a 12-week exercise program, reductions in body fat via DXA scan, BIA, and InBody (whole body fat, whole body fat %, trunk fat, leg fat, body fat %, hip width at maximum girth, body fat mass, and percent body fat) were observed. Additionally, increases in lean mass (Appendicular Lean Mass (ALM)/height², Appendicular Lean/height², ALM index, and lean body mass), whole body mass measured by DXA, basal metabolic rate, water retention (extracellular water and total body water), and skeletal muscle mass were observed. Correlation analysis revealed BMI can be used as a surrogate measure for whole body mass measured by DXA (r=0.91, p<0.001) and can be utilized to measure change over time. Skinfold sum showed moderate correlation for measuring subcutaneous fat mass by DXA (r=0.45, p=0.0096). Conclusions: In our population of breast cancer survivors, a 12-week exercise program did show various changes in body composition according to InBody, DXA, and BIA measurements. DXA and InBody measurements were feasible, making it readily available for patients. Additionally, simple measures such as BMI and skinfold sum showed moderate to strong correlation with DXA measurements, validating use of these measurements in routine practice. Clinical trial information: NCT04013568.