Free-floating right heart thrombus can be seen in 4% to 18% of patients presenting with acute pulmonary embolism. A 76-year-old man was admitted to the intensive coronary care unit due to resting dyspnea and pleuritic pain of sudden onset, raising a high suspicion of acute pulmonary embolism. A recent coronary angiogram showed a 50% stenosis in the proximal left anterior descending coronary artery. He had diabetes and hypertension for more than 10 years, but no history of venous thromboembolism. Bed-side transthoracic echocardiography revealed dilated right heart chambers, and a huge (78x12 mm) mobile mass in the inferior vena cava. We witnessed the migration of the thrombus from the inferior vena cava to the right atrium. The thrombus then totally lodged in the right atrial cavity and protruded into the right ventricle. Surgical removal of the thrombus was decided. However, during induction of anesthesia, cardiac arrest developed. All resuscitation efforts including open heart massage were unsuccessful. The thrombotic material removed from the right atrium was 150 mm in length. Pathological examination showed the mass to be a thrombus.