We describe the case of an 80-year-old man admitted to the hospital for the first time with chest pain and progressive respiratory difficulty. Radiographic findings of the chest showed a large, cystic mediastinal mass from the jugulum to the diaphragm. The patient's history revealed bypass operation for a benign esophageal stricture 30 years ago. During the hospital stay, clinical symptoms resolved within 48 hours without specific treatment. Seven days after admission a chest roentgenogram showed almost complete regression of the tumor, which was supposed to be a mucocele of the colon bypassing the esophagus.