Background: Obesity is known to increase risk of postoperative complications for patients undergoing general and cardiovascular surgery. The impact of obesity on risk of complications among women with breast cancer is less understood. Access to medical care may be associated with management of obesity and its related conditions, as well as in the diagnosis and treatment of cancer. Thus, it is unknown what the association between obesity and risk of short-term complications and readmission after breast cancer surgery is when controlled for access to medical care. We aimed to study the association of obesity with risk of general and breast-specific complications among women with breast cancer in the U.S. Military Health System which provides access to care for all eligible Department of Defense beneficiaries with little financial barriers. Methods: Women aged 18 and older with non-metastatic invasive breast cancer between 2001 and 2014 and who underwent breast conservation or total mastectomy without immediate reconstruction were included. Obesity status was determined by the presence of International Classification of Diseases (ICD)-9th edition diagnosis codes corresponding to body mass index (BMI) >=30 in the medical encounter data prior to breast cancer diagnosis. General (e.g. myocardial infarction, pneumonia) and breast surgery-specific complications, breast reoperation, and hospital readmissions were assessed in the 30-day postoperative period. Modified Poisson regression was used to compare the risk of the outcomes between women with and without obesity expressed as adjusted risk ratios (ARRs) with 95% confidence intervals (CIs). Results: The study included n=1,094 women with obesity (mean age 57.5 +/-10.9 years) and n=6,741 women without obesity (mean age 55.9 +/-13.0 years) receiving surgery without immediate reconstruction for stage I-III breast cancer. Overall, the frequency of general complications was 2.5% among women with obesity and 1.9% among women without obesity (Chi-square p=0.22). The risk of general complications was not statistically different by obesity status (ARR=0.95, 95% CI=0.61, 1.49 for obese vs. non-obese) in multivariable analyses with adjustment for potential confounders. The frequency of breast-specific complications was 8.9% among women with obesity and 6.3% among women without obesity (Chi-square p=0.03). In Poisson models, women with obesity had a 29% higher risk of breast complications (ARR=1.29, 95% CI=1.03, 1.61) relative to women without obesity. For specific complications, the risk among women with obesity was higher relative to those without obesity for surgical site infection (ARR=1.56, 95% CI=1.10, 2.21) and lymphedema (ARR=1.71, 95% CI=1.02, 2.88) and not statistically different for hematoma or seroma after adjusting for patient, tumor, and surgery characteristics. There were no statistical differences in risk of reoperation (ARR=0.93, 95% CI= 0.80, 1.10) or readmission (ARR=1.04, 95% CI=0.79, 1.36). Conclusion: In the universal Military Health System, women with obesity had an increased risk of site-specific postoperative complications following breast cancer surgery, especially surgical site infection and lymphedema. Our results highlight the importance of weight management in breast surgical oncology care and indicate that efforts are needed to identify effective interventions which may reduce both obesity and risk of postoperative complications among women with breast cancer.