The authors report two operated cases of floating left atrial thrombus. Both patients had severe mitral stenosis, atrial fibrillation and dilatation of the left atrium. In the first case, the patient had no anticoagulant therapy; in the second case, the observance of anticoagulant therapy was irregular. Neither patient complained of syncope. The diagnosis was made in both cases by two-dimensional echocardiography which, in addition to confirming the severe mitral stenosis, showed a round, free-floating intra-atrial mass, rebounding from wall to wall and repeatedly engaging itself into the mitral orifice. The risk of prolonged engagement of the thrombus in the mitral orifice, causing syncope or sudden death, justifies urgent surgery associating thrombectomy and treatment of mitral stenosis every time this problem is encountered.