We report on the case of spontaneous rupture of an On-X-pure pyrolytic carbon mechanical valve prosthesis implanted seven years earlier, in a mitral position, at our hospital. The patient was admitted with valvular dysfunction and acute pulmonary edema requiring emergency surgery (prosthesis replacement); the absence of a leaflet was confirmed intraoperatively. The patient presented severe respiratory failure, which prolonged the postoperative period. A CT scan showed that the migrated leaflet was located in the aortic bifurcation with no apparent arterial lesion. Four months later, once the patient had recovered, laparotomy and aortotomy were performed in order to retrieve the leaflet, which was found to have become included (neoendothelized) in the aortic wall without compromising the latter's integrity or obstructing the blood flow. A subsequent CT scan confirmed the persistence of the leaflet in its initial position. The literature review highlights two singular facts: 1) this is the second published case of the escape of a leaflet from an On-X prosthesis (the first patient died); 2) this is the first case in which a laparotomy was performed to retrieve the leaflet but finally a decision was made to leave it in situ. Seven months later, the patient remained asymptomatic.