A 63-year-old female, who had been suffering from idiopathic interstitial pneumonia (IIP) since two years and four months previously, was admitted with general fatigue, body weight loss, bloody sputum and progression of dyspnea. Chest roentgenogram on admission showed diffuse coarse ring shadows in the right middle and lower lung fields and in the left middle lung filed. Chest CT clearly showed honeycombing in bilateral dorso-basal segments of the lung. Serum CA19-9 level was elevated, and continued to increase during her clinical course. Sputum cytology was positive. Autopsy showed papillary adenocarcinoma of the lung (alveolar cell type). Immunohistochemical study of lung tissue showed positive CA19-9 staining on the surface of adenocarcinoma cells.