The matrix metalloproteinase (MMP) enzymes MMP-9, have relevance to chronic structural airway changes in asthma, which can be generated by structural and inflammatory cells, and have the ability to degrade proteoglycans and thus potentially enhance airway fibrosis and smooth muscle proliferation through their ability to release and activate latent, matrix-bound growth factors. Immunostaining for MMP-9 was undertaken in acetone-fixed and glycolmethacrylate-embedded endobronchial biopsy specimens obtained by fibreoptic bronchoscopy under local anaesthesia. The findings from 30 asthmatic subjects were compared with those from 18 chronic obstructive pulmonary disease (COPD) subjects and 10 healthy controls. Meanwhile, pulmonary function test and airway responsiveness were performed. Immunoreactivity for MMP-9 was assessed by an image analysis system. The biopsy specimens from asthmatic subjects contained significantly more eosinophils (P < 0.001) than those from COPD subjects, and healthy control did not contain eosinophils. MMP-9 immunoreactivity could be identified in endobronchial biopsy specimens from all the asthmatic subjects and 40% ofthe COPD subjects, but could not be identified in healthy controls. Gelatinase B (MMP-9) immunoreactivity was located in bronchial epithelium and extracellular matrix in submucosa, prominent in denuded epithelium. The immunohistochemical score for MMP-9 was significantly correlated with eosinophilic number in bronchial mucosa. FEV1% predicted FEV1/FVC (%) (r = 0.52, 0.41, 0.37, respectively P < 0.01 did not correlate with PD20 FEV1 from asthmatic subjects. MMP-9 is expressed by bronchial epithelium and may be a important factor for eosinophil infiltraed into airway from asthma subjects.