Background-The impact of aortic prosthesis-patient mismatch (P-PtM) on long-term survival is unclear. . P-PtM was severe in 66 patients (17%), moderate in 168 (43%), and not hemodynamically significant in 154 (40%). Patients with severe P-PtM had a significantly larger body surface area (PϽ0.0001), higher mean gradient (PϽ0.0001), lower preoperative (PϽ0.0001) and postoperative (PϽ0.0001) ejection fractions, and lower stroke volume (PϽ0.0001) and more often received a 19-mm prosthesis (Pϭ0.0008) than patients with moderate or no hemodynamically significant mismatch. For patients with severe mismatch, 5-year survival rates (72Ϯ6%) and 8-year survival rates (41Ϯ8%) were significantly less than for patients with moderate mismatch (80Ϯ3% and 65Ϯ5%; Pϭ0.026) or no hemodynamically significant mismatch (85Ϯ3% and 74Ϯ5%; Pϭ0.002). On multivariate analysis after adjustment for other predictors of outcome, severe mismatch was associated with higher mortality (hazard ratio 2.18; 95% confidence interval 1.24 to 3.85; Pϭ0.007) and higher incidence of congestive heart failure (hazard ratio 3.1; 95% confidence interval 1.3 to 7.4; Pϭ0.009) than no hemodynamically significant mismatch. Conclusions-Severe P-PtM is an independent predictor of higher long-term mortality and congestive heart failure in patients with small St Jude Medical aortic valve prostheses. For patients undergoing AVR who are at risk of severe mismatch, every effort should be made to use a larger prosthesis or to consider a prosthesis with a larger EOA. Methods and Results-Between