This trial is a multinational, multicenter, randomized, open-label, active-controlled clinical trial comparing the safety of aspirin versus vitamin K antagonists (warfarin) as oral antithrombotic therapy during the initial 6 months following tissue valve replacement (aortic or mitral).

Inclusion Criteria:

1. Aged ≥ 19 years with successful surgical bioprosthetic valve replacement
2. Bioprosthetic heart valve in aortic or mitral valve positions, for a day postoperatively
3. The patient or guardian agrees to the study protocol and the schedule of clinical follow-up, and provides informed, written consent, as approved by the appropriate Institutional Review Board/Ethical Committee of the respective clinical site.

Exclusion Criteria:

1. Mechanical valve in any heart valve position
2. Previous hemorrhagic stroke within the last 3 months
3. Clinically overt stroke within the last 3 months
4. Current need for anticoagulation (i.e. atrial fibrillation, deep vein thrombosis)
5. Unstable postoperative conditions at index point (i.e. requirement mechanical hemodynamic support, disabling complications, stroke [ischemic, embolic], hemorrhagic stroke)
6. Hypersensitivity to the main component or constituents of aspirin or vitamin K antagonist
7. Pregnant or lactating women