To provide information concerning a possible biologic role of nitric oxide (NO) in smoking-related emphysema, we performed immunohistochemical studies in lung tissue from control subjects and patients with mild and severe emphysema. We studied the presence of inducible and endothelial NO synthases (iNOS and eNOS, respectively) and determined nicotinamide diphosphate (NADPH) diaphorase activity. Patients with severe emphysema showed lower percentages of iNOS-and eNOS-positive alveolar macrophages in situ than did patients with mild emphysema. In patients with both iNOS and eNOS immunoreactivity in macrophages, the majority of the macrophages expressed either iNOS or eNOS, whereas only a minority of the macrophages showed iNOS and eNOS immunoreactivity simultaneously. Immunoreactivity for eNOS in endothelial and/or bronchiolar epithelial cells and NADPH diaphorase activity in macrophages and in endothelial, epithelial, and smooth muscle cells were similar in the three studied groups. The expression of eNOS in macrophages suggests that eNOS plays an additional role, besides iNOS, in the NO housekeeping in inflammatory processes in pulmonary tissue. We suggest that NO might have a protective role in maintenance of structural integrity of pulmonary tissue after smoke-induced damage.