Typical imaging characteristics of benign mediastinal cysts include the following: (1) a smooth contour and round or oval shape; (2) homogenous attenuation of variable density depending on fluid protein and debris content; (3) lack of postcontrast enhancement or enhancement limited to the outer wall or margin, ie, the inflamed wall of a cyst may enhance but the contents do not; and (4) lack of infiltration into adjacent stuructures.1 In this case, a lesion meeting all of the above criteria was identified on the CT scan. The location between the seventh vertebral body and posterior margin of the heart led to a differential diagnosis of cystic lesions arising from structures normally found in the middle or posterior mediastinum (Table 1). The thin, well-circumscribed walls, homogeneous attenuation, and lack of enhancement on CT made cystic degeneration of a solid tumor, such as a mediastinal carcinoma, abscess, or nerve sheath tumor, unlikely, because such lesions typically exhibit thick, irregular, enhancing walls. Necrotic lymph nodes may present with either thick and irregular or thin and smooth enhancing margins with low central attenuation sometimes making definitive diagnosis on CT difficult. Pancreatic pseudocysts may extend from beneath the diaphragm into the mediastinum but can usually be traced to their origin and occur in the context of pancreatitis. Congenital lesions may include foregut duplication, neuroenteric, and pericardial cysts. Given the typical cystic appearance of this lesion, and its location and immediate proximity to a thin ascending tubular structure, the diagnosis of a thoracic duct cyst was considered most likely prior to resection. Thoracic duct cysts are uncommonly reported in the medical literature and share imaging characteristics of other benign cysts.2,3 The initial clue to diagnosis is finding the cyst in a location normally traversed by the typically diminutive thoracic duct.