A 50-year-old perimenopausal woman presented with complaints of intermittent mild dizziness. She had a high-stress job and a history of hyperthyroid thyroiditis, for which she had undergone ablation therapy 7 years earlier. A thorough head and neck examination revealed no abnormalities, and unenhanced magnetic resonance images (MRIs) of the brain (Figure 1) were interpreted as unremarkable. The patient saw a dentist for routine care and was told that she had classic temporomandibular joint syndrome. However, treatment with ibuprofen, heat compresses, and massage therapy provided no relief. Ten weeks after the MRI was performed, acute facial swelling arose, and the patient palpated an intraoral submucosal mass at the base of the zygoma. The physical finding was confirmed by an otolaryngologist, and a computed tomogram (CT) of the neck was obtained. The CT revealed a 2-cm heterogeneously enhancing mass along the anterior aspect of the left mandibular coronoid process (Figure 2). The underlying coronoid process was eroded (Figure 3), and the mass was inseparable from the masseter and temporalis muscles. The retromaxillary fat pad was invaded by the mass. There was no clinical or radiographic evidence of lymphadenopathy, and the patient denied a history of trauma, oral discharge, dysphagia, recent infection, or irradiation. The mass, which was barely visible as a focus of slightly increased T2 signal on the MRIs that had been obtained 2 months earlier, had dramatically increased in size. A complete blood cell count, electrolytes, thyroid-stimulating hormone, thyroxine, triiodothyronine uptake, free thyroxine index, total estrogen, progesterone, follicle-stimulating hormone, and luteinizing hormone levels were within the normal range, and the patient's only medication was levothyroxine sodium.