Vascular malformations of the mediastinum including mediastinal lymphangioma are exceedingly rare in adults. A resected case of mediastinal cystic lymphangioma (cystic hygroma) that presented with acute onset of retrosternal chest pain is reported in a 45-year-old man. Cystic hygromas are benign cysts frequently discovered incidentally on chest radiograph. Although computed tomography scan provides helpful information about the size, density, and site of the cysts, it cannot establish a precise diagnosis concerning its nature. Complete removal of the cyst is the treatment of choice and allows precise diagnosis on histologic examination. However, surgical excision is sometimes difficult, owing to the size and extension of the cysts, infiltrating mediastinal planes, enveloping great vessels, and displacing mediastinal organs without invasion. The difficulty of completely eradicating certain cysts explains cases of insidious progression with compressive recurrence. The subject is succinctly reviewed.