Between March 1970 and the end of December 1986, 180 porcine valve recipients underwent reoperation at our institution because of bioprosthetic failure. Of these patients, 126 (70.0%) had mitral bioprosthesis replacement (MBR), which was necessitated because of primary tissue failure in 107 cases, prosthetic valve endocarditis in eight cases, and paravalvular leakage in eleven cases. Forty-six patients (25.5%) had aortic bioprosthesis replacement (ABR), owing to primary tissue failure in 37 cases, endocarditis in six cases, and paravalvular leakage in three cases. The remaining eight patients (4.5%) all underwent double (mitral and aortic) bioprosthesis replacement (DBR) because of primary tissue failure. In a total of 152 patients who underwent reoperation for primary tissue failure, the hospital mortality was 6.5% for the MBR group, 8.0% for the ABR group, and 0% for the DBR group (mean, 6.5%). In those 14 patients who required reoperation owing to endocarditis, the hospital mortality was 75% for the MBR group and 50% for the ABR group (mean, 64%). In another 14 patients who underwent reoperation because of paravalvular leakage, the hospital mortality was 9% for the MBR group and 0% for the ABR group (mean, 7%). Morphologic studies of the explanted valves revealed that tissue calcification is the most frequent cause of primary tissue failure in Hancock-valve recipients. Calcification leads to cusp stiffening with stenosis or to cuspal and commissural rupture with incompetence. Usually, primary tissue failure causes progressive dysfunction that allows for elective reoperation, which is associated with a low mortality; rarely, however, porcine xenograft failure can occur acutely and require emergency operation, which is associated with a significantly higher risk. Such operation was necessary in nine (6%) of our patients with primary tissue failure and resulted in four deaths (44%); conversely, elective reoperation was done in 143 patients with other primary tissue failure (94%) and resulted in six deaths (4%) (p<0.001). Our long-term experience with the Hancock bioprosthesis confirms that this valve's durability is limited. Therefore, we believe that the indications for the use of porcine bioprostheses should be restricted, until consistent data become available concerning the durability of the new generation of prosthetic valves.