Pulmonary fibrosis resulting from increased accumulation of various extracellular matrices is a prominent feature in chronic progressive lung diseases. Heat shock protein 47 (HSP47) is a collagen-binding stress protein known to have a specific role in the intracellular processing of procollagen molecules as a collagen-specific molecular chaperone in various organs. Possible involvement, however, of HSP47 in relation to increased deposition of collagens in fibrotic lung diseases is not yet known. In this study, we investigated the expression of HSP47 in various pulmonary fibrotic diseases. Formalin-fixed, paraffin-embedded lung sections from 17 autopsies of patients with various pulmonary fibrotic diseases, e.g., organizing pneumonia, interstitial pneumonia, idiopathic pulmonary fibrosis, and diffuse alveolar damage, were stained with monoclonal antibodies for alpha-smooth muscle actin, vimentin, CD68, Type III collagen, and HSP47. The extent of staining was graded semiquantitatively. Five control lung sections were also simultaneously studied. The fibrotic lung sections, in comparison with the control sections, had more interstitial cells positive for alpha-smooth muscle actin and fibroblasts positive for vimentin; we also saw increased infiltration of CD68-positive macrophages. For HSP47, in comparison with the control lung sections, markedly increased immunostaining was always noted in the fibrotic lung sections in association with increased accumulation of Type III collagen in the fibrotic masses. By double immunostaining, colocalization of collagens and HSP47 was noted in the regions of pulmonary fibrosis, and HSP47-expressing cells were found to be mainly alpha-smooth muscle actin-positive interstitial cells. From the above observations, we concluded that overexpression of HSP47 might play an important role in the excessive assembly/synthesis of collagens and could thereby contribute to the fibrosis found in pulmonary fibrotic lung diseases.