A 27-year-old man was admitted with chest pain and cough in January 1999. Chest radiograph on admission showed a widened tracheal bifurcation. Computer tomography on admission showed a low density mass located at the tracheal bifurcation. Magnetic resonance imaging of the chest showed a well defined mass with isointensity on T1-weighted images, and high intensity on T2-weighted images. Laboratory data on admission showed mild inflammatory findings and a high level of Sialy1 Lewis X-i antigen (SLX) in serum. Thoracotomy revealed a cystic mass and pathologically, the cyst wall was lined with bronchial epithelium which showed no malignancy. The level of SLX in the cystic fluid was elevated, and immunohistochemical staining of the cystic epithelium was positive for SLX. After resection of the cyst, the level of SLX in serum decreased. This represents a rare case of bronchogenic cyst with a high level of SLX in serum and cystic fluid.