Background. The discovery of a large left atrial mass through echocardiography obliges the clinician to perform a differential diagnosis to distinguish tumor from thr ombus. In fact, magnetic resonance imagery could be useful to identify the mass but it could not distinguish tumor from organized thrombus. Certainly, surgery is the best solution for a successful diagnosis.Case report. The 67 year old woman was admitted to cardiology department with dyspnea, orthopnea, palpitations, and fatigue. Anamnesis: 10 years of atrial fibrillation and type 2 diabetes and 15 years with arterial hypertension. By the time of addressing, the patient has been administering anticoagulan ts for several months with warfarin while maintaining INR 2. Physical examination revealed an irregular pulse, at a rate of 110 beats/min. The electrocardiogram revealed an atrial fibrillation with rate 150 100 b/min. The chest X ray pulmonary congestion . TTE revealed a severe mitral stenosis (GPmax 33 mm/hg, area 0,6 cm 2 ) with third degree mitral regurgitation and left atrial mass (50*36 mm), third degree tricuspid regurgitation. Left atrium was enlarged (67*84 mm), severe pulmonary arterial hypert ension. These findings were confirmed by TEE. The preoperative coronarography showed neovascularization among the mass and huge fistula from the circumflex artery in the tumour mass and left atrium. We strongly suspected a vascular tumor, especially myxoma . Preoperative decision was made to perform cardiac MRI “ shaped mass formation, fixed to the upper rear wall of the LA, 7 cm long, massive thrombus. Cardio surgical intervention was performed: MV prosthesis MDT “Hancock II ultra” N29, complex plas tic repair of TrV, removing the massive thrombus from the LA. After surgery, the patient had uncomplicated recovery. Conclusions. Atrial mass management will be based on clinical history (mitral stenosis, permanent atrial fibrillation) and echocardiograph ic data. If atrial mass persists during treatment with anticoagulants, cardiac MRI and coronarography are useful for diagnosis. However, the final diagnosis is established during cardiac surgery.