OBJECTIVE To emphasize a potentially lethal condition that is virtually impossible to diagnose preoperatively. METHODS Case report with review of the literature. METHODS University Hospital. METHODS The patient requiring urgent surgery for heart failure related to severe aortic stenosis and mild mitral stenosis with poor ventricular function. The patient was elderly and suffered from atrial fibrillation. METHODS Preoperative transesophageal echocardiography followed by mitral valve repair and aortic valve replacement. METHODS Clinical outcome and pathological results. RESULTS Although preoperative TEE demonstrated no left atrial appendage abnormality. After cardiac manipulation prior to the institution of cardiopulmonary bypass a large left atrial mural thrombus was mobilized from the atrial wall and was free floating in the left atrium. CONCLUSIONS For high risk patients TEE should be applied intraoperatively to avoid undiagnosed left atrial clot dislodgement.