A 35-year-old Japanese man's routine chest radiography revealed an abnormal opacity. Chest computed tomography and magnetic resonance imaging showed a 5.5 cm in dia. cystic tumor located at the left anterior mediastinum. The tumor was suspected to be an asymptomatic thymic cyst, and we chose observation for the tumor. At the 3-year follow up, the cystic tumor had gradually enlarged to 7.5 cm in dia. and we thus performed a surgical resection via left video-assisted thoracic surgery. An immunohistochemical analysis showed that the cystic tumor was not a thymic cyst but rather a mediastinal cystic lymphangioma. Mediastinal cystic lymphangiomas are very rare, and they are difficult to diagnose preoperatively. Complete surgical resection is proposed for the treatment of such tumors.