A large intracardiac mass is a rare condition and one with an extremely high risk of haemodynamic and embolic complications. Urgent surgical excision is the treatment of choice, and the histological examination reveals the exact nature of the mass, usually a myxoma or a thrombus. We present the case of an 80-year-old woman, with a history of atrial fibrillation, who was admitted because of a seriously impaired level of consciousness, and fever. A large cerebral infarct and a urinary tract infarction were diagnosed. On the transthoracic echocardiogram a giant, free-floating mass was detected in the left atrium, transiently obstructing the mitral valve orifice. Based on the features of the mass and patient's history, it was considered more likely to be a thrombus rather than a tumour. Given the patient's extremely unfavourable neurological status, cardiac surgery was considered to be contraindicated and the patient was administered unfractionated heparin intravenously. Unfortunately, after a few hours the patient suffered a cardiac arrest and died.