Mediastinal lymphangioma is primarily a benign lesion and the majority of the cases are found incidentally. These lesions account for approximately 1% of all mediastinal tumors. Here we present a giant mediastinal cystic mass in a 35-year-old female who was presented with severe respiratory distress. On the plain chest radiography and CT scan, a massive left pleural effusion with large parasternal and mediastinal lymphadenopathy was seen. Thoracentesis was performed and 400 cm³ of clear fluid was drained from the left hemithorax. However, a subsequent CT scan with contrast and the same technique 40 days later showed a large cystic mass in the mediastinum protruding to the right and left hemi thoraces. The giant cystic mass was resected via right and left anterior thoracotomies. Histopathological examination revealed a diagnosis of lymphangioma. The patient has been alive and without tumor recurrence and has been followed for 2 years.