An increased incidence of lung cancer and epithelial metaplasia or hyperplasia which is felt to be as a precursor of cancer, has been reported in patients with idiopathic pulmonary fibrosis (IPF). In this study, carcinoembryonic antigen (CEA) in bronchoalveolar lavage (BAL) fluid was measured in 53 control patients, 31 patients with sarcoidosis, 10 patients with hypersensitivity pneumonitis, 16 patients with primary lung cancer and 26 patients with histologically confirmed IPF. High ratio of CEA to albumin (Alb), exceeding mean + 2SD of nonsmoking control patients, were found in 8 (25%) out of 32 smoking control patients, 4 (44%) out of 9 nonsmoking patients with IPF, 8 (62%) out of 13 smoking patients with IPF, 3 (75%) out of 4 smoking patients with IPF and lung cancer and 13 (81%) out of 16 patients with primary lung cancer, although BAL was performed at the noncancerous parts of the lung in the cases of lung cancer. Furthermore, it was confirmed that CEA increased in BAL fluid in these subjects were different from nonspecific cross-reacting antigen (NCA) which was detectable in the normal lung. Thus we consider that the increase of CEA/Alb ratio in BAL fluid is a possible marker of these early histological disorders in the lung, and also suggests a greater risk of malignant change in the clinical course of IPF.