Cytokeratin 19 (CK19) is a specific cytoskeletal structure for simple epithelia, including bronchial and alveolar epithelial cells (BAEC). Since CK19 is abundant in alveolar epithelial cells, and could be released from injured alveolar epithelium in idiopathic pulmonary fibrosis (IPF), we investigated the levels of CK19 fragments in the bronchoalveolar lavage fluids (BALF) of 16 patients with idiopathic pulmonary fibrosis (IPF) and 12 patients with sarcoidosis using enzyme-linked immunoassay. There were also 19 control subjects (10 asymptomatic smokers and nine non-smokers). BALF from the non-smokers as well as the asymptomatic smokers contained few CK19 fragments (0.2+/-0.2, 1.3+/-0.5 pg ml(-1) respectively). There were significantly high levels of CK19 in the BALF from patients with IPF (7.3+/-1.4 pg/ml; P<0.01 vs. control non-smoker). Even if the levels of CK19 were expressed as relative to the albumin concentration, significantly increased levels of CK19 fragments were noted in BALF from patients with IPF. However, these levels were not found in BALF from patients with sarcoidosis. Importantly, levels of CK19 fragment in BALF were significantly correlated to the number of neutrophils (r = 0.791, P<0.001) and eosinophils (r = 0.771, P<0.001) but not to that of macrophages or lymphocytes in BALF from IPF patients. Our results suggest the usefulness of CK19 measurement in BALF for assessing the presence of bronchiolo-alveolar epithelial injuries in idiopathic pulmonary fibrosis.