The study was designed to evaluate the effects of moderate prosthesis-patient mismatch (defined as 0.65 cm(2)/m(2) <indexed effective orifice area ≤ 0.85 cm(2)/m(2)) on midterm outcomes after isolated aortic valve replacement with a 17-mm St. Jude Medical Regent valve in a large series of patients, and to determine if these effects are influenced by patient confounding variables.