Background: Enhanced Recovery after Surgery formastectomy has resulted in increased use of outpatient same-daymastectomy (SDM). Whether SDM leads to increased readmissions or reoperations is not well documented. This study examines national data to compare outcomes of SDM to an overnight stay. Methods: We analyzed the American College of Surgeons National Surgical Quality Improvement Program Participant Use Data File from 2016 to 2018 for all mastectomy cases. Cases with a length of stay (LOS) >1 day were excluded. Cases were then categorized into 2 LOS cohorts: SDM vs 1-day LOS. Results: A total of 22,642 cases (80.8% 1-day LOS vs 19.2% SDM) were identified for the final analysis. Patients in the 1-day LOS group were more likely to be older (57.9 vs 54.0 years, P<0.01), be female (98.0% vs 79.8%, P<0.01), and have greater comorbidity (38.1% vs 30.7% American Society of Anesthesiologists classification 3 or 4, P<0.01) compared to the SDM group. Multivariate analysis demonstrated no difference in risk for 30-day wound complications between the SDM and 1-day LOS groups. The risks for 30-day medical complications (1.60 odds ratio [OR], 95% CI 1.06-2.42, P=0.02), reoperations (1.46 OR, 95% CI 1.17-1.81, P<0.01), and readmissions (1.60 OR, 95% CI 1.25-2.05, P<0.01) were higher in the 1-day LOS group. Even after excluding patients undergoing reoperation on the day of surgery, the risk for reoperations (2.3% vs 3.3%, P<0.01) remained higher in the 1-day LOS group. Characteristics associated with 1-day LOSwere hypertension, steroid use, diabetes, dyspnea, dependent functional status, bilateral procedures, and breast reconstruction. Conclusion: We demonstrate that SDM is a safe procedure, with no increase in risk for 30-day postoperative complications. Appropriate patients should be offered SDM. [ABSTRACT FROM AUTHOR]