The authors describe the computed tomography (CT) and magnetic resonance (MR) imaging findings of a 69-year-old woman and a 69-year-old man with a nonfunctional mediastinal parathyroid cyst. In the described cases, unenhanced CT showed homogeneous areas of water density, and unenhanced MRI showed homogeneous areas that were isointense to cerebrospinal fluid, reflecting their serous fluid contents. Both cysts were located posterior to the left lower pole of the thyroid gland with an extension to the superior mediastinum, either anterior or posterior to the left brachiocephalic vein. CT and MR imaging findings of parathyroid cysts are nonspecific, and they are often difficult to differentiate from other cystic lesions located in the lower neck or in the superior mediastinum. However, a parathyroid cyst should be considered when radiologic images demonstrate its characteristic location, posterior to the thyroid gland, with an extension to the superior mediastinum.