Although abnormalities of alveolar fibrin turnover have been reported to play a role in the development of idiopathic pulmonary fibrosis (IPF), the pathophysiological relevance remains unclear. We therefore investigated the localization of tissue factor (TF) and fibrin deposition in patients with IPF using immunohistochemistry and compared the results with those from patients who had interstitial pneumonia associated with systemic sclerosis (IP-SSc) and idiopathic bronchiolitis obliterans with organizing pneumonia (BOOP). Expression of TF-mRNA was also assessed, using in situ hybridization with a digoxigenin-labeled cRNA probe. In patients with IPF, IP-SSc, and idiopathic BOOP, the TF antigen was positively stained in type II pneumocytes and in some alveolar macrophages. The fibrin antigen was stained in the type II pneumocytes and the adjacent area. Tissue factor-mRNA was expressed in the type II pneumocytes and in some alveolar macrophages. Neither TF antigens nor TF-mRNA were detected in the normal lung. These results indicate that type II pneumocytes are a major source of TF, suggesting that TF production in these cells is closely related to fibrin deposition in the lungs of people with these diseases.