Moderate mitral stenosis, associated to left atrium enlargement and atrial fibrillation, can accelerate thrombo embolic formation. If not found quickly, medical treatment may not be enough. We report a case of 64 years-old women with dyspnea and palpitation related to a moderate mitral valve stenosis, atrial fibrillation and enlargement of left atrium containing a large thrombus. Despite a well conduced anticoagulation treatment, she presented an episode of syncope with a thrombus that was more developed than previously. She underwent mitral prothesis, thrombectomy and reduction of left atrium with good outcome.