Mucoepidermoid carcinoma of the thymus is an extremely rare malignant mediastinal neoplasm, and to our knowledge, only 13 cases have been reported. We report a case of mucoepidermoid carcinoma of the thymus that was seen in a 53-yr-old man with right chest pain. Chest CT scan showed a huge, cystic mass having a focal solid portion with direct invasion of the adjacent anterior chest wall and pericardium in the anterior mediastinum. Mucoepidermoid carcinoma of the thymus should be included in the differential diagnosis for masses of the anterior mediastinum associated with extensive cystic changes, although the carcinoma is exceedingly rare.