Purpose: Reduction mammaplasty has transitioned into a largely outpatient procedure in the United States. Following planned outpatient procedures, patients may still be admitted for additional inpatient care, incurring clinical and economic burden. Prior literature has not explored the preoperative and perioperative determinants of extended lengths of stay (LOS) after breast reduction surgery. Methods: Patients who underwent scheduled outpatient reduction mammaplasty were identified via current procedural terminology code from the 2013 to 2021 National Surgical Quality Improvement Program databases. The primary outcome was extended LOS, defined as an LOS greater than 1 day. The most significant predictor variables were identified through bivariate association, and a binary logistic regression model was used to characterize predictive associations (p < 0.05). Results: In this study, 33,924 patients were included in the final cohort of planned outpatient reduction mammaplasty cases. Among them 325 (1.0%) patients had extended LOS. Concurrent liposuction, body contouring, and increased operative time were the most significant predictors of extended LOS (p < 0.001), followed by older age, higher body mass index, bleeding disorder, history of diabetes, higher American Society of Anesthesiologists class, and White race (p < 0.05). When adjusted for other confounding variables, extended LOS was also a significant predictor of increased risk of postoperative complications after discharge (OR: 1.85, 95% confidence intervals: 1.27-2.69, p = 0.0012). Conclusion: Extended LOS after planned outpatient reduction mammaplasty is associated with specific comorbidities, and is a significant predictor of postoperative complications following hospital discharge. (c) 2024 British Association of Plastic, Reconstructive and Aesthetic Surgeons. Published by Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.