A 54-year-old man was admitted as an emergency following an episode of syncope and severe dyspnea. The clinical presentation and laboratory data strongly suggested pulmonary embolism, so intravenous thrombolytic therapy was performed. Immediately following the initiation of thrombolytic therapy, the patient experienced cardiopulmonary arrest, but he was successfully resuscitated. Ascending venography of the left lower limb demonstrated a large intraluminal thrombus which was apparently floating between the femoral vein and inferior vena cava. In addition to the anticoagulation therapy, a Greenfield filter was implaced intravenously to prevent recurrent embolization.