An 83-year-old woman was referred to our hospital to examine for an infiltration shadow in the right lower lobe with progressive bronchorea Computed tomography showed an infiltration lesion with the longest diameter of 10 cm in the right lower lobe and a tumor with the longest diameter of 3 cm in the middle lobe. Serum level of carbohydrate antigen (CA) 19-9 markedly increased to 37,670 U/ml over a period of 3 months. The pathologic study obtained by a transbronchial tumor biopsy revealed a mucinous adenocarcinoma The patient underwent video-assisted thoracoscopic right middle and lower bi-lobectomies with nodal sampling. Postoperative course was uneventful Pathologic study revealed an adenocarcinoma with mixed subtypes, predominantly composed of mucinous bronchiolo-alveolar cell carcinoma (BAC). Immunohistochemical study showed CA19-9 positivity in the apical surface of some tumor cells and diffuse patterns of other tumor cells. Postoperative course was uneventful and serum CA19-9 levels decreased within the normal range. Clinico-pathologic features of the lung cancer patients with serum elevation of CA19-9 and CA19-9 positivity in the cancer cells was discussed. CA19-9 can be an useful tumor marker in the selected patients with mucinous BAC.