In interstitial lung diseases, deposition of extracellular matrix (ECM) in alveoli and degradation of ECM lead to pulmonary structural remodeling. The changes in ECM and the localization of matrix metalloproteinases (MMPs) and a tissue inhibitor of metalloproteinases (TIMP) in the lung tissues of patients with bronchiolitis obliterans organizing pneumonia (BOOP) and idiopathic pulmonary fibrosis (IPF) were investigated. Immunohistochemical analysis for the detection of fibronectin, collagen-I, -III, and -IV, smooth muscle actin, MMP-1 (interstitial collagenase), -2 (gelatinase A), and -9 (gelatinase B), and TIMP-2, and in situ hybridization for the detection of MMP-9 mRNA were performed. Western blotting of lung tissue homogenates was performed for MMP-2 and MMP-9. The gelatinolytic activities of the homogenates were also determined using gelatin zymography. Fibronectin and collagen-I, -III, and -IV were detected in the intra-alveolar fibrosis in addition to the interstitium of these diseases. MMP-1, MMP-2, MMP-9, and TIMP-2 were detected in the regenerated epithelial cells covering intra-alveolar fibrosis. Myofibroblasts in intra-alveolar fibrosis in BOOP showed predominant reaction for MMPs, and they ultrastructurally appeared to be phagocytosing collagen fibrils, and those of IPF showed a predominant reaction for TIMP-2. New vascularization in intra-alveolar fibrosis was exclusively observed in cases of BOOP, and the endothelial cells were positive for MMP-2. Western blotting showed the existence of a latent form of MMP-9 and latent and active forms of MMP-2, and gelatin zymography revealed that the ratio of active/latent forms of MMP-2 in BOOP is significantly larger than that in the control lungs. Predominant MMPs in BOOP may constitute the mechanism of reversibility of fibrotic changes in this disease. TIMP-2 in myofibroblasts in IPF may contribute to the stable ECM deposition and the irreversible pulmonary structural remodeling.