We describe a patient with mitral stenosis and severely enlarged left atrium. Transthoracic echocardiography showed a false image of intraatrial thrombus, whereas transesophageal echocardiography showed massive spontaneous left atrial contrast. Intraoperative transesophageal echocardiography was performed. During cardioplegic arrest the contrast was enhanced, but it gradually and completely cleared 15 minutes after cardiopulmonary by-pass arrest. Transesophageal echocardiography is a useful technique for the study of intraatrial masses and may bring a new dimension to tissue characterization studies.