A 54-year-old woman presented with a 12-month history of painless, progressive swelling of the left side of her neck. Physical examination revealed a fluctuant, nontender mass in the left supraclavicular region. The skin overlying the mass was unremarkable. The patient had been diabetic for the last 15 years and had cardiac problems. She had no surgical history, and the results of her other physical examinations and laboratory tests were normal. Chest radiography revealed no abnormalities. Computed tomography showed a well-circumscribed, lobulated 5 × 10-cm cystic mass (attenuation values, 10-20 Hounsfield units) that extended from the thyroid cartilage to the superior mediastinium and deviated the trachea to the right side.