Cytokeratin 19 fragment (CK19) levels in serum have already been documented as a useful tumour marker for lung cancer. In the present study, it was hypothesized that CK19 may be increased in the serum and epithelial lining fluid of the respiratory tract from patients with pulmonary fibrosis. CK19 was measured in the serum and bronchoalveolar lavage fluid (BALF) of patients with pulmonary fibrosis and the correlation between CK19 levels and clinical parameters evaluated. Nineteen patients diagnosed with idiopathic pulmonary fibrosis (IPF), eight with pulmonary fibrosis associated with a collagen vascular disorder (PF-CVD), seven patients with acute interstitial pneumonia (AIP), and 10 normal smokers as a control group were studied. CK19 levels in sera of patients with IPF and patients with PF-CVD were significantly increased compared to those of normal smokers. CK19 levels in sera of patients with AIP were significantly increased compared to those of other groups. CK19 values in the BALF of patients with pulmonary fibrosis were significantly elevated compared to those of normal smokers. CK19 values in sera charged according to the progression or improvement of the acute lung injury. Immunohistochemical study using pulmonary tissues obtained from patients with AIP demonstrated that the hyaline membrane and proliferating type II pneumocytes were stained by anti-human cytokeratin 19 antibody. These data demonstrated that the measurement of cytokeratin 19 fragment is a useful parameter to evaluate the activity of lung epithelial cell damage and repair.