We report a rare case of spontaneous rupture of idiopathic thymic abscess into the pleural cavity. A 64-year-old woman was admitted to hospital with pleuritic retrosternal chest pain. Chest roentgenograms disclosed a small amount of bilateral pleural effusion, the examination of which exposed a sterile serous exudate with a markedly increased CA-125 level. Chest computed tomography revealed a large anterior mediastinal cystic mass with bilateral pleural effusions. Following complete resection of the mass, the histological examination revealed cavitary lesion with necrotic thymic tissue and inflammatory infiltrate surrounded by fibrous wall. The immunohistochemical staining for CA-125 displayed strong positivity at the Hassall's corpuscles. Cyst fluid also revealed a highly elevated CA-125 level. Her serum CA-125 concentration two months after surgery had fallen to 28 IU/L. She is now doing well without recurrence of the cyst five months after surgery.