Cystic hygroma is benign overgrowth of the lymphatic vessels and is usually located in the neck, but rare in the mediastinum. This vascular tumor may appear in any portion of mediastinum, but seems to be most common anteriorly. This is a report of one case of congenital mediastinal cystic hygroma in a 25 years old man, especially in the thymic location which was surgically removed successfully with good postoperative result and confirmed histopathologically. [KTCS 1982;3:299-302]