A 42 years old man with evident distortion in the lower 2/3 of the right hemithorax, area in which he presents a compression syndrome and radiologically a bulky opacity with tumoral aspect that shifts the heart and mediastinum to the left, is presented. The thoracotomy reveals a giant cystic formation containing approximatively 7 l. of rough magma into which hair, cartilaginous and bone tissue are identified. After their removal the cystic wall (1 cm thick) is excised except for an implantation area of 4/8 cm adherent to the large vessels and pericardium. The postoperative evolution is uneventful. The histopathological examination confirms the diagnostis of MCD identifying derivatives from all 3 embryonic sheets.