Prospective Randomized On-X Versus SJM Evaluation Trial

Inclusion Criteria: 1. The patient requires an isolated mitral or isolated aortic valve replacement. (Patients undergoing coronary artery bypass and / or concomitant repair of mitral or tricuspid valves are eligible.) 2. The patient is a candidate for receipt of a mechanical heart valve. 3. The patient (or legal guardian) has signed a study-specific informed consent form agreeing to the randomization, data collection and follow-up requirements. 4. The patient can be having a re-operative procedure, so long as the previous prosthetic valve is explanted and the patient does not become a double valve implantation patient. Exclusion Criteria: 1. The patient is not a candidate to receive a mechanical heart valve. 2. The patient already has a prosthetic valve other than the valve(s) being replaced at this time. 3. The patient requires a tricuspid valve replacement. 4. The patient is enrolled in another investigative study or trial.