We report the discovery at transthoracic echocardiography of a very large, sessile formation in the left atrium in a 77-year-old woman with permanent non-valvular atrial fibrillation who did not receive anticoagulant therapy. After six weeks of anticoagulation by subcutaneous low-molecular-weight heparin, a control echocardiogram demonstrated an almost complete resolution of the mass without systemic embolisation. This case highlights the success, without complications, of a medical anticoagulant therapy for giant left atrial thrombus.