The intraoperative use of two-dimensional transesophageal echocardiography has proved effective in the evaluation of left ventricular function after heart operations, in the assessment of adequacy of valve replacement or repair techniques, and in the detection of intracardiac air bubbles before discontinuation of cardiopulmonary bypass. We report here a patient in whom the presence of a tumor mass in the left leaflet of the aortic valve, which was missed at preoperative transthoracic echocardiogram and would have most likely been the cause of systemic embolization, was diagnosed by two-dimensional transesophageal echocardiographic monitoring. We hope that this experience may support the use of two-dimensional transesophageal echocardiography during cardiac surgery.