We report 12 cases of well-differentiated thyroid carcinoma that invaded the trachea. In all of these cases, we performed a hemithyroidectomy, including the isthmus, with an accompanying neck dissection and resection of the trachea. Six of 12 patients experienced hemoptysis, and a diagnosis of tracheal invasion was made preoperatively in nine patients by tracheal endoscopy and computed tomography. Histologic diagnosis was confirmed by a preoperative biopsy in one case only. An end-to-end anastomosis of the trachea was performed in five patients, an anastomosis between the cricoid cartilage and the trachea was performed in five patients, and an anastomosis between the thyroid cartilage and the trachea was performed in two patients. One patient with a recurrence of tumor died of tracheal bleeding 11/2 years later. One patient died of massive gastrointestinal bleeding postoperatively. The remaining ten patients have been doing well from three months to five years two months postoperatively.