In chronic inflammatory diseases, cells are recruited but may also derive from local proliferation. In normal bronchial epithelium, under 5% of cells are in cycle but in asthma and chronic bronchitis, proliferation may occur. Cycling cells can be identified by immunohistochemistry using PC10 monoclonal antibody (Proliferating Cell Nuclear Antigen, PCNA). We enumerated PCNA-positive cells (labeling index = LI) in bronchial biopsies of 11 healthy non-smokers (HNS), seven healthy smokers (HS), 30 non-smoking asthmatics (NSA), six smoking asthmatics (SA) and 18 chronic bronchitics (CB). Twenty non-small cell lung cancer patients were used as positive control subjects. Ciliated and secretory cells were PCNA-negative. Basal cells were PCNA-positive in one of the 11 HNS (LI = 0.18 +/- 0.60), none of the seven HS, two of the 30 NSA (LI = 0.05 +/- 0.20), two of the six SA (LI = 2.4 +/- 4.3) and 11 of the 18 CB (LI = 12 +/- 20). In smokers, PCNA positivity correlated with tobacco consumption (Rho = 0.62, p < 0.0008) and in patients with chronic bronchitis, with the degree of metaplasia (tau = 0.815, p < 0.0001). The submucosa of most subjects showed no PCNA immunoreactivity. These findings suggest that the bronchial mucosa of nonsmokers is not hyperproliferative, even in asthmatics. Tobacco smoking increases PCNA immunoreactivity, possibly leading to the metaplasia of chronic bronchitis.