During the last fourteen years, 377 unileaflet tilting-disc prosthetic valves (Björk-Shiley and Medtronic Hall) have been used for single or multiple valve replacements with and without concomitant coronary artery by-pass grafting. In the past five years, five instances of disc immobilization (three in the mitral and two in the aortic position) occurred either at the time of weaning from cardiopulmonary bypass or immediately thereafter. When the implanted site of the prosthetic valve was the mitral position, reexploration in 2 patients revealed chordal remnants in the subannular area stuck between the disc occluder and the valve ring, thereby immobilizing the disc. In the third instance, the free movement of the disc was impeded by the left ventricular myocardium. In the aortic position, an unraveled suture impacted between the disc occluder and the valve ring immobilized the disc in 1 patient. In the other patient, the cause of the malfunction could not be determined at the time of reexploration. The 1 death among these 5 patients was directly related to the malfunction of the prosthesis. The mechanism, recognition, treatment, and prevention of this catastrophic malfunction of tilting-disc valves are discussed.