Primary leiomyosarcomas of the thyroid gland are rare. We present the case of a 65-year-old woman with a rapidly enlarging neck mass for 2 months. The preoperative differential diagnosis included medullary thyroid cancer, anaplastic thyroid cancer, and primary versus metastatic sarcoma. The patient underwent total thyroidectomy, bilateral central neck dissections, and cervical thymectomy; she is currently being treated with ifosfamide and adriamycin. We review the literature on leiomyosarcoma of the thyroid, including the differential diagnoses, pathology, and alternative treatment strategies, including surgery and adjuvant therapy.