Background: Same-day discharge total hip arthroplasty (THA) has grown in utilization although concerns exist regarding early complications and catastrophic events. We sought to compare the risk of complications and catastrophic events for same-day and inpatient stay THA. Methods: A cohort study was conducted using Kaiser Permanente's total joint replacement registry. Primary elective THA were identified (2017-2018). Propensity scoreeweighted Cox proportional hazards regression was used to evaluate risk for 90-day incident events, including emergency department (ED) visit, unplanned readmission, cardiac complication, deep infection, venous thromboembolism (VTE), and mortality, by in-hospital length of stay: same-day vs 1-2-night inpatient stay. Results: The study sample comprised 13,646 THA, 6033 (44.1%) with a same-day discharge. Median daysto-events for same-day vs inpatient was 11 vs 12 for ED visit, 23 vs 20 for readmission, 38 vs 12 for cardiac complication, 28 vs 24 for deep infection, 14.5 vs 23.5 for VTE, and 7 vs 35.5 for mortality. In propensity scoreeweighted models, same-day discharge THA had a lower risk for 90-day ED visit (HR = 0.82, 95% CI = 0.72-0.94), readmission (HR = 0.75, 95% CI = 0.61-0.92), and cardiac complication (HR = 0.60, 95% CI = 0.47-0.76), compared with inpatient stay THA; no difference was observed for deep infection (HR = 1.59, 95% CI = 0.81-3.12), VTE (HR = 0.90, 95% CI = 0.52-1.58), or mortality (HR = 0.81, 95% CI = 0.27-2.40). Conclusion: We observed a lower or no difference in risk for complications and catastrophic events after same-day THA than an inpatient stay. Catastrophic events were more likely to occur early in the 90-day period, but an inpatient stay did not preclude events. (C) 2021 Elsevier Inc. All rights reserved.