There are variable reported effects of patient-prosthesis mismatch (P-PM) on outcome. It was hypothesized that the adverse effect attributed to P-PM is actually due to left ventricular diastolic dysfunction (DD) in patients with small hearts. The aim of this study was therefore to determine the association among P-PM, DD, and outcomes. Doppler echocardiography was performed in 156 patients after aortic valve replacement. In vivo effective orifice areas for each prosthesis type and size were obtained from published references values of normally functioning prostheses. P-PM was identified from the predicted indexed orifice area, obtained by dividing the effective orifice area by body surface area. DD was classed as normal, delayed relaxation (prolonged deceleration time for age), or increased left atrial pressure (increased E/E′ ratio, left atrial enlargement, short deceleration time). Events (cardiac-related hospitalizations and all-cause mortality after aortic valve replacement) were determined over a median follow-up periods of 3.5 years (interquartile range 2.1 to 5.7). P-PM was found in 91 patients (58%). Of the patients with P-PM, no DD was present on postoperative echocardiography in 15 patients (16%), delayed relaxation in 35 (39%), and increased left atrial pressure in 41 (45%). There were 61 total events (18 deaths and 43 hospitalizations): 4 (7%) in the no-DD group, 26 (42%) in the delayed relaxation group, and 31 (51%) in the increased left atrial pressure group. DD (p = 0.034) but not age (p = 0.09), the left ventricular ejection fraction (p = 0.60), or the presence of mismatch (p = 0.20) was associated with events. In conclusion, P-PM was associated with 14% mortality and a 39% composite event rate over 2-year follow-up. Events were significantly associated with DD.