BACKGROUND Subclinical leaflet thickening and reduced leaflet motion of bioprosthetic aortic valves have been documented by four-dimensional computed tomography (CT). Whether anticoagulation can reduce these phenomena after transcatheter aortic-valve replacement (TAVR) is not known. METHODS In a substudy of a large randomized trial, we randomly assigned patients who had undergone successful TAVR and who did not have an indication for long-term anticoagulation to a rivaroxaban-based antithrombotic strategy (rivaroxaban [10 mg] plus aspirin [75 to 100 mg] once daily) or an antiplatelet-based strategy (clopidogrel [75 mg] plus aspirin [75 to 100 mg] once daily). Patients underwent evaluation by four-dimensional CT at a mean (±SD) of 90±15 days after randomization. The primary end point was the percentage of patients with at least one prosthetic valve leaflet with grade 3 or higher motion reduction (i.e., involving >50% of the leaflet). Leaflet thickening was also assessed. RESULTS A total of 231 patients were enrolled. At least one prosthetic valve leaflet with grade 3 or higher motion reduction was found in 2 of 97 patients (2.1%) who had scans that could be evaluated in the rivaroxaban group, as compared with 11 of 101 (10.9%) in the antiplatelet group (difference, -8.8 percentage points; 95% confidence interval [CI], -16.5 to -1.9; P=0.01). Thickening of at least one leaflet was observed in 12 of 97 patients (12.4%) in the rivaroxaban group and in 33 of 102 (32.4%) in the antiplatelet group (difference, -20.0 percentage points; 95% CI, -30.9 to -8.5). In the main trial, the risk of death or thromboembolic events and the risk of life-threatening, disabling, or major bleeding were higher with rivaroxaban (hazard ratios of 1.35 and 1.50, respectively). CONCLUSIONS In a substudy of a trial involving patients without an indication for long-term anticoagulation who had undergone successful TAVR, a rivaroxaban-based antithrombotic strategy was more effective than an antiplatelet-based strategy in preventing subclinical leaflet-motion abnormalities. However, in the main trial, the rivaroxaban-based strategy was associated with a higher risk of death or thromboembolic complications and a higher risk of bleeding than the antiplatelet-based strategy.