An 82-year-old female had undergone aortic valve replacement with a 23-mm St. Jude Medical X-Cell porcine bioprosthesis (XCB), a glutaraldehyde-fixed valve subjected to a decellularization process. More than 13 years later, she required a repeated operation because of XCB structural failure. Pathologic examination of an unused XCB as control showed that many interstitial cells were still present in the porcine cusps. The XCB explant exhibited gross mineralization on radiograph, fairly well-preserved fibrosa, lack of interstitial and endothelial cells, scattered empty spaces with plasma insudation, and calcific deposits. Ultrastructural examination revealed calcium deposition on collagen fibers and cell debris. Partial cusp decellularization observed in an XCB was not effective in preventing mineralization and most likely should be associated with other antimineralization treatments as well as methods favoring cell repopulation in vitro or in vivo.