Thrombosis of mechanical aortic valve prosthesis is a rare but life-threatening complication. In most reported cases, inadequate anticoagulation or cessation of anticoagulation is the cause of prosthesis thrombosis. The case of a 70-year-old male patient hospitalized for severe dyspnoea is presented. Although the patient was under continuous anticoagulation, thrombosis of the 16 years previously implanted mechanical 31-sized aortic valve prosthesis was diagnosed. Emergency surgery was performed and postoperative course was uneventful. Patients with large size prostheses should have closer anticoagulation monitoring, even after many years of event-free postimplant course.