A 54-year-old male, who had been involved in a traffic accident one week earlier, suddenly died in hospital without any warning. Autopsy revealed right hemothorax due to rupture of a subpleural hematoma of the right lung which was connected to a false aneurysm of the descending aorta. It is likely that the false aneurysm began to grow just after the traffic accident, forming an aortopleural fistula due to preceding adhesion. This led to the formation of a hematoma in the right subpleural space, which ruptured one week after the traffic accident. The present case indicates that the natural course and pathogenesis of traumatic false aneurysm depends upon the preceding local anatomical circumferences.