Clinical use of the stentless bioprosthesis has not yet been accepted world-wide. Experimental studies are scarce. In a study in growing pigs, 23 aortic valve replacements were performed (7 stented bioprostheses (STB), 10 stentless bioprostheses (SLB) and 6 cryopreserved homografts (CAH)). Valves were studied macroscopically and microscopically after explantation. Five animals died between 1.5 and 4 months after implantation. Two STBs showed extreme calcific degeneration. A third STB animal died suddenly: this STB showed only minimal leaflet calcification. One SLB was stenosed with a large fibrin deposit in one cusp, a second SLB showed slight cusp calcification and three were paracommissural tears in one cusp. In all other animals the valves were explanted after 5 to 6.5 months. All STBs showed severe calcific degeneration. Five SLBs showed mild calcific degeneration, while three others were unaltered after 6 months. All CAHs were free of calcific degeneration, three were perfect, two had a tear in a commisure and another was prolapsed with a fibrin degeneration. The speed and extent of valve degeneration were less than in the STB group. The stentless design is an important contribution which may result in a higher durability of bioprosthetic valves.