Heat shock protein 47 (HSP47) is a collagen-specific molecular chaperone that has been shown to play a major role in the processing and/or secretion of procollagen. However, the knowledge on which cells are actually synthesizing HSP47 in the lung parenchyma in pulmonary fibrosis was only limited. The aim of the present study was to investigate the localization of HSP47 messenger ribonucleic acid (mRNA) in normal lung and in the lungs of mice in bleomycin-induced pulmonary fibrosis, using in situ hybridization. For the purpose, ICR mice were intravenously injected with 10 mg/kg per day of bleomycin for five consecutive days. The lung cells expressing HSP47 mRNA were identified in control (saline alone) and bleomycin-treated mice by in situ hybridization. The signal for HSP47 mRNA was markedly increased in bleomycin-treated lungs compared with that of controls. HSP47 mRNA was localized in alpha-smooth-muscle-actin-positive myofibroblasts, surfactant-protein-A-positive type II pneumocytes, and F4/80-positive macrophages in the active fibrotic areas. These results suggest that these cells may synthesize procollagen in the fibrotic process of bleomycin-treated lungs through upregulation of HSP47 mRNA and play an important role in fibrogenesis.