Undifferentiated high grade pleomorphic sarcoma of the thyroid gland is very rare. A 70-year-old woman was referred to our hospital for evaluation of a rapidly growing mass in the right lobe of the thyroid gland. At the first visit, she had no symp-toms. Fine-needle aspiration cytology revealed an atypia of undetermined significance. She revisited our hospital after 17 days. At that time the mass had become more enlarged and she complained of dyspnea. The next day, surgery was performed and the diagnosis was undifferentiated high grade pleomorphic sarcoma of the thyroid. The patient died 119 days after sur-gery.