THE PARTNER TRIAL: Placement of AoRTic TraNscathetER Valve Trial

Inclusion Criteria Cohort A 1. Patients must have co-morbidities such that the surgeon and cardiologist Co-PIs concur that the predicted risk of operative mortality is ≥15% and/or a minimum STS score of 10 2. Patient has senile degenerative aortic valve stenosis with echocardiographically derived criteria: mean gradient \>40 mmHg or jet velocity greater than 4.0 m/s or an initial aortic valve area of \< 0.8 cm2 3. Patient is symptomatic from his/her aortic valve stenosis, as demonstrated by NYHA Functional Class II or greater 4. The subject or the subject's legal representative has been informed of the nature of the study, agrees to its provisions and has provided written informed consent as approved by the IRB of the respective clinical site 5. The subject and the treating physician agree that the subject will return for all required post-procedure follow-up visits Cohort B All candidates for Cohort B of this study must meet #2, 3, 4, 5 of the above criteria and 6. The subject, after formal consults by a cardiologist and two cardiovascular surgeons agree that medical factors preclude operation, based on a conclusion that the probability of death or serious, irreversible morbidity exceeds the probability of meaningful improvement. Specifically, the probability of death or serious, irreversible morbidity should exceed 50%. Exclusion Criteria 1. Evidence of an acute myocardial infarction ≤ 1month before the intended treatment. 2. Aortic valve is a congenital unicuspid or bicuspid valve; or is non-calcified 3. Mixed aortic valve disease (aortic stenosis and aortic regurgitation with predominant aortic regurgitation \>3+). 4. Any therapeutic invasive cardiac procedure performed within 30 days of the index procedure, (or 6 months if the procedure was a drug eluting coronary stent implantation). 5. Pre-existing prosthetic heart valve in any position, prosthetic ring, or severe (greater than 3+) mitral insufficiency.