Purpose: The purpose of this study was to evaluate the impact of the use of external radiation therapy (ERT) in terms of survival and compliance in patients with anaplastic thyroid carcinoma. Materials and Methods: The medical records of 17 patients with anaplastic thyroid carcinoma treated with ERT between 1993 and 2002 were retrospectively reviewed. ERT was administered after surgery in 14 patients and after a biopsy in three patients. Among the 14 patients who had undergone surgery, nine underwent a curative resection and five underwent a palliative resection. Six patients had associated well-differentiated thyroid carcinomas and 14 patients were diagnosed with a tumor size exceeding 5 cm. The radiation dose ranged from 6∼70 Gy (median dose, 37.5 Gy). Eleven patients completed the planned course of ERT, whereas six patients did not. The follow-up period ranged from 1∼104 months (median, 5 months; mean, 20 months). Results: Five patients started the ERT without the presence of a