Histologic and scanning and transmission electron microscopic studies were made to determine the frequency of occurrence of endothelial cells in 49 porcine valve bioprostheses removed from 43 patients 2 days to 113 months (average 35 months) after implantation. Endothelial cells were found in none of 17 bioprostheses in place for less than 1 year, in 5 (23 percent) of 22 in place for 1 to 5 years and in 7 (70 percent) of 10 in place for longer than 5 years. The 12 bioprostheses in which endothelial cells were present had been implanted in the atrioventricular position (7 of 32 in the mitral position and 5 of 6 in the tricuspid position) for 21 to 113 months (average 71). Endothelial cells were not found in any of 11 bioprostheses implanted in the semilunar position (9 in the aortic position and 2 in pulmonary conduits); however, all but one of these bioprostheses had been in place for less than 5 years. Endothelial cells were concentrated along the basal regions of the cusps. These cells did not grow in direct contact with valve collagen, but were attached to fibrin, thrombi or fibroelastic host tissue (fibrous sheath) on the valve surfaces. The growth of endothelial cells and associated fibrous tissue may serve to increase the structural stability of bioprosthetic valve cusps, which may be of functional importance several years after implantation when the porcine connective tissue may have undergone significant deterioration.