In the present study, the expression of NOX4 and its correlation with the ASM hypertrophy/hyperplasia, clinical pulmonary functions, and the expression of transforming growth factor β (TGF-β) in the ASM of COPD small airways were investigated by semiquantitative morphological and/or immunohistochemistry staining methods. Here, NOX4 is linked to chronic obstructive pulmonary disease.