OBJECTIVE Our study compares late mortality and valve-related morbidities between nonelderly patients (aged <65 years) undergoing stented bioprosthetic or mechanical valve replacement in the aortic position. METHODS We identified 1701 consecutive patients aged <65 years who underwent aortic valve replacement between 1992 and 2011. A stented bioprosthetic valve was used in 769 patients (45%) and a mechanical valve was used in 932 patients (55%). A stepwise logistic regression propensity score identified a subset of 361 evenly matched patient-pairs. Late outcomes of death, reoperation, major bleeding, and stroke were assessed. RESULTS Follow-up was 99% complete. The mean age in the matched cohort was 53.9 years (bioprosthetic valve) and 53.2 years (mechanical valve) (P=.30). Fifteen additional measurable variables were statistically similar for the matched cohort. Thirty-day mortality was 1.9% (bioprosthetic valve) and 1.4% (mechanical valve) (P=.77). Survival at 5, 10, 15, and 18 years was 89%, 78%, 65%, and 60% for patients with bioprosthetic valves versus 88%, 79%, 75%, and 51% for patients with mechanical valves (P=.75). At 18 years, freedom from reoperation was 95% for patients with mechanical valves and 55% for patients with bioprosthetic valves (P=.002), whereas freedom from a major bleeding event favored patients with bioprosthetic valves (98%) versus mechanical valves (78%; P=.002). There was no difference in stroke between the 2 matched groups. CONCLUSIONS In patients aged <65 years, despite an increase in the rate of reoperation with stented bioprosthetic valves and an increase in major bleeding events with mechanical valves, there is no significant difference in mortality at late follow-up.