We report herein the rare case of a 20-year-old man in whom a mediastinal lymphangioma was incidentally detected by a chest roentgenogram taken during a routine health examination. Both computed tomography and magnetic resonance imaging confirmed a mass measuring 3 x 7 cm in diameter in the left anterior mediastinum. A thoracoscopic exploration was done, which confirmed a diagnosis of mediastinal lymphangioma, and 3 days later a sternotomy was performed. However, the tumor could not be completely extirpated due to partial invasion. Following the thoracoscopic procedure, a chylous discharge developed which was difficult to treat conservatively and he continued to drain 700-1,000 ml of chyle daily 2 weeks following the tumor extirpation. Therefore, a right thoracotomy with ligation of the thoracic duct was performed which resolved the chylothorax. The patient remains well without any regrowth of the regional tumor 9 months after his operation.