Host factors might influence the fate of tissue prostheses. We have analyzed the tissue failure phenomenon in mitral and aortic porcine bioprostheses explanted from the same patient so as to avoid the bias of host factors. The 50 bioprostheses of this study were explanted from 25 patients reoperated on because of malfunctioning due to primary tissue failure. Bioprostheses explanted were photographed, radiographed and observed in polarizing light. All lesions analyzed received a score on the basis of morphological criteria. No difference was observed in the occurrence of pannus, tears or cells infiltrates. Calcifications occurred more extensively in mitral position (p=0.0031). Creep of the stent was significantly greater in mitral position (p=0.0324). Since the model of our study removed other individual factors, we may conclude that there is evidence for an earlier and more extensive calcific degeneration in the mitral than in the aortic position.