A 59-year-old man had undergone hemodialysis for 16 years because of chronic renal failure. The patient had taken an aspirin therapy (100 mg per day) for 4 years because of his history of brain infarction. He had a 3-week history of increasing back pain. A chest computed tomographic scan demonstrated a mass in the upper mediastinum. The mass was located among the superior vena cava, trachea, and ascending aorta. Two weeks later, magnetic resonance imaging revealed that the mass had become slightly smaller. The patient's symptom also disappeared gradually. Follow-up imaging showed that the mass had resolved completely. The clinical and imaging findings corresponded with a case of spontaneous mediastinal hematoma presenting as a mass.