Catheter-based closure of the left atrial appendage is an alternative to oral anticoagulation for stroke prevention in patients with atrial fibrillation. The effectiveness of this strategy, as compared with physician-directed best medical care, in patients at high risk for stroke and bleeding is unknown. In this multicenter randomized trial conducted in Germany, we assigned patients with atrial fibrillation and a high risk of stroke and bleeding to undergo left atrial appendage closure or to receive physician-directed best medical care (including direct oral anticoagulants, if eligible). The primary end point, tested for noninferiority, was a composite of stroke (ischemic or hemorrhagic), systemic embolism, major bleeding, or cardiovascular or unexplained death, assessed in a time-to-event analysis. The noninferiority margin was a hazard ratio of 1.3. A total of 912 adult patients underwent randomization. The primary end-point analysis included 446 patients who were assigned to undergo left atrial appendage closure (device group) and 442 who were assigned to physician-directed best medical care (medical-therapy group). The mean (±SD) age was 77.9±7.1 years; 38.6% of the patients were women, the mean CHA Among patients with atrial fibrillation at high risk for stroke and bleeding, left atrial appendage closure was not noninferior to physician-directed best medical care with regard to a composite end point of stroke, systemic embolism, major bleeding, or cardiovascular or unexplained death. (Funded by the German Center for Cardiovascular Research; CLOSURE-AF ClinicalTrials.gov number, NCT03463317.).