A rare case of pulmonary embolism resulting from a biatrial mobile thrombus that was lodged in the patent foramen ovale is reported here. A 78-year-old man was admitted to this institution complaining of sudden onset of dyspnea. Arterial blood gas analysis showed remarkable hypoxemia. Transthoracic and transesophageal echocardiography showed pulmonary arterial hypertension as well as right atrial and ventricular dilation. A highly mobile snakelike thrombus was readily apparent in the right and left atria. The left atrial thrombus was thought to have come by way of the patent foramen ovale as a result of the right-to-left shunt induced by elevated right atrial pressure and was suspected to have come from a venous thrombiscast. The biatrial thrombus dissolved within 24 hours, probably because of spontaneously activated fibrinolysis. We diagnosed a pulmonary embolism, and the patient was treated vigorously with such anticoagulation agents as intravenous heparin sodium and orally administered warfarin potassium. Therapeutic results were monitored by repeated echocardiographic examination.