The study hypothesizes that rate of thromboembolic complications of the On-X prosthesis is reduced as compared to the SJM prosthesis.

The study is a multi-centre, randomized trial that will sequentially enroll up to 500 eligible patients in each group from up to 10 participating study centres internationally to test the hypothesis.

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.