A 44-year-old woman was found to have an abnormally high concentration of CA19-9 in serum. Five years later, she underwent a right upper lobectomy for an abnormal shadow on a chest roentgenogram, because it was thought to indicate a malignant tumor. A chest CT scan revealed a cyst with fluid in the right upper lobe. During the 5 years before the lobectomy, the serum CA19-9 concentration rose when the cyst fluid volume increased, and fell when the cyst fluid volume decreased. Before the operation, the serum CA19-9 concentration was very high and the cyst was swollen. The cyst wall was found to be lined with ciliated columnar epithelial cells and to contain hyaline cartilage and smooth muscle, which led to the diagnosis of bronchial cyst. The surface epithelium and cyst fluid were positively stained by a monoclonal antibody against CA19-9. The serum CA19-9 concentration decreased and was within the normal range 3 months after the operation.