Bioprosthetic valves are used for patients who's conditions are unfavorable for anticoagulant therapy. A 72-year-old woman underwent operation for severe mitral stenosis and mild regurgitation in our cardiovascular surgery clinic four years ago 27 mm (this is highly unusual) St Jude to have a bioprosthesis mitral valve implanted. Surgeons chose bioprosthesis instead of a mechanical valve due to her inadequate international normalized ratio (INR) control. The patient recovered control INR six months after surgery. 100 mg acetylsalicylic acid was administered once daily. The patient attended our cardiology polyclinic for exercise induced dyspnea. Electrocardiography was in sinus rythm upon examination for a period of 12 months. In the physical examination (when) a diastolic suffle heard upon mitral focus. Transthoracic echocardiography showed a mobile mass in the left atrium. In transesophagial echocardiography we saw a mobile mass extending from bioprosthetic mitral valve level to the atrium base (Fig. 1). Cardiac MRI indicated a 6×2.5 cm thrombus (astenix) on the posterior wall of the left atrium (Fig. 2). The patient was transferred to cardiovascular surgery for operation. The thrombus was cleared (Fig. 3) and the left atrial appendage was closed. 12 days after the operation, the patient was discharged from the hospital with cure. The INR ratio was calibrated to 2.5-3.5 using warfarin therapy.