 This is a longitudinal, randomized randomization to occur at the 3-month follow-up study comparing the On-X valve on low dose anticoagulation test group to concomitant control groups of On-X valves receiving standard Coumadinaspirin therapy, and also to FDA objective performance criteria OPC for heart valve replacement. It is a multicenter study consisting of up to 50 centers in the United States and Canada enrolling no more than 1200 patients 200 in each of 6 groups. There are three test arms of the study: low risk aortic valve replacement, high risk aortic valve replacement, and mitral valve replacement. Each arm has an equivalent control. Test therapies are: low risk aortic valve replacement - aspirinPlavix, high risk aortic valve replacement - Coumadin at INR of 1.5 to 2.0 plus aspirin, and mitral valve replacement - Coumadin at an INR of 2.0 to 2.5 plus aspirin. Follow-up will run for up to 8 years in each patient. Each arm is independent and the low risk aortic and high risk aortic arms are completed. The low risk aortic arm was closed early resulting in a reduction of the estimated enrollment to 1000. The high risk arm is completed with FDA review and this arm had 375 enrollees. The mitral arm continues to enroll with a planned enrollment of 400. 