Of 581 aortic allografts implanted since 1967, 421 were analyzed for structural deterioration. This series is unique in that it includes patients from the early allograft experience. All allografts were cleanly procured, antibiotic sterilized, and either stored at 4 degrees C for up to 8 weeks or frozen to liquid nitrogen temperatures with cryopreservation to preserve the viable cusp fibroblasts. There were 25 frozen mounted aortic valves with a median time to valve failure of 12.1 years, which was not significantly different from the 12.5-year period for 114 fresh free-sewn aortic valves. The median time to valve failure was 6.6 years for 90 fresh-mounted aortic valves and 8.6 years for 192 fresh-mounted mitral valves (p = 0.05). The difference between all mounted and unmounted grafts was significant (p = 0.0001). In all groups, viable fibroblasts were present in specimens explanted up to 5 years after the operation. All specimens returned after more than 10 years were almost totally acellular. Evidence of increased collagen, suggesting that the fibroblasts survive implantation and then gradually die, was present in all specimens. This series suggests that durability of the unmounted viable allograft for aortic valve replacement is greater than for other types of tissue valves. Pre-mounted allografts for aortic or mitral valve replacement have a median survival of 8 years and are not more durable than other tissue valves.