We have experienced two cases of complicated teratoma in anterior mediastinum. One case, 1-year-old female, has a huge anterior mediastinal teratoma involving almost all of the left pleural cavity with atelectasis of the left lung. The other case, 12-year-old female, has a anterior mediastinal teratoma with fistula between the teratoma and the bronchus of anteromedial basal segment of left lung. The bronchus has a bronchiectatic change with cell infiltration. We performed exploratory thoracotomy with complete excision of tumor mass in both cases and with additional resection of anteromedial basal segment of the left lung due to inflammation and necrosis in latter case. The patients had uneventful postoperative courses and were discharged in good condition. Histopathologic diagnosis of these were benign teratoma consist of skin, skin appendages, cartilage, connective tissue, and gut.[KTCS 1982;2:169-173]