Mediastinal lymphangioma is a rare benign lesion and mostly asymptomatic and usually found accidentally in childhood. We experienced a case of huge mediastinal cystic mass which occupied the whole left hemithorax in a 2-year-old male patient presenting cough and tachypnea. The tumor was completely resected by means of posterolateral thoracotomy. Pathological diagnosis confirmed cavernous lymphangioma. There is no evidence of tumor recurrence for 24 months postoperatively.