A 31-year-old woman underwent elective transthoracic echocardiography for paroxysmal atrial fibrillation, which showed a large left atrial mass. A two-dimensional echocardiogram showed a large left atrium mass. Subsequent transesophageal echocardiography showed a 3 cm x 3 cm circular mass with smooth contours. The differential diagnosis included myxoma versus thrombus, but because of morphology and mobility of the mass, it was difficult to differentiate one from the other. The patient had been on chronic coumadin therapy for paroxysmal atrial fibrillation. An operation was performed and the intracardiac mass resected. On pathologic examination the mass was diagnosed as an organized thrombus. A brief review of the literature has been presented to discuss the differential diagnosis of the mass in the left atrium.