Objective. To analyze the peculiarities of surgical treatment of a mitral stenosis, complicated by massive thrombosis of left atrium. Маterials and methods. The group analyzed, operated in the Institute, consisted of 344 patients. Thrombosis of left atrium was considered a massive, when thrombotic masses have occupied no less than one third of its volume, not mentioning an auricle of atrium. Results. Hospital lethality after change of a mitral valve have constituted 4.2% and directly depended on from a degree of the left atrium thrombosis (р < 0.05). After open mitral comissurotomy hospital lethality was not observed, witnessing the expediency of the thrombosis matrix extraction. Conclusion. During the operation for a massive thrombosis of left atrium it is important to remove a maternal base of thrombotic bed and to eliminate the left atrium auricle, what lowers essentially the risk for lethality and thromboembolic complications on a hospital stage. Doing preoperative computed tomography of head and abdominal organs constitutes obligatory condition for exclusion of a hidden thromboembolism occurrence.