This morphologic study (X-ray examination of gross specimens, histologic study and histochemical staining) compares two groups of explanted left-sided bioprosthetic valves: group I, 6 valves with normal cusp function and group II, 10 valves with significant dysfunction. Implantation periods ranged from 26 to 79 months. A computerized descriptive statistical method (principal component analysis) is used to analyze the qualitative results. Although qualitatively identical alterations are observed in both groups, the findings in the deep layers of the cusps of severe collagen breakdown, intensive fibrin penetration and various degrees of calcification are restricted to group II. Other findings of interest in both groups include amyloid deposits (four cases) and layering of fusiform host cells on the cusp surface (three cases). The computerized study shows that individuals of one clinical group are morphologically different from those of the other. Mechanical stress may contribute to surface alterations early after implantation, while further collagen breakdown and macrophagic activity result in deep penetration of plasma components and fibrin. Subsequent calcification is likely to be dystrophic rather than metabolic. Colonization of the cuspal surface by endothelial cells after long-term implantation of bioprosthetic valves expresses a new type of relation between host and bioprosthesis.