In order to reveal modulation of the number of pulmonary neuroendocrine cells (PNEC) in interstitial lung diseases and to clarify significance of cell proliferation activity in occurrence of PNEC, we counted airway PNEC of the patients of idiopathic interstitial pneumonia, secondary interstitial pneumonia and control lungs, and compared the number of PNEC with airway Ki-67 labeling. The lung tissue samples were obtained by video-assisted thoracoscopic surgery from 22 patients with usual interstitial pneumonia (UIP), 7 with non-specific interstitial pneumonia (NSIP), 8 with chronic hypersensitivity pneumonia (CHP), 13 with collagen vascular disease (CVD), and were compared with age-matched control lungs. The tissues were immunostained for chromogranin A and for Ki-67. Average incidence of bronchiolar PNEC in normal, UIP, NSIP, CHP, CVD lungs was 0.169%, 0.348%, 0.326%, 0.175% and 0.201%, respectively, and average Ki-67 labeling index in them was 0.241%, 1.186%, 1.605%, 1.058%, and 2.353%, respectively. And, in UIP lungs, PNEC incidence or Ki-67 labeling index was different according to pathological lesions. Thus, PNEC increase in the bronchiole of UIP, and the incidence of PNEC varies according to degree of activity of epithelial cell proliferation probably related to epithelial cell injury. Moreover, enhanced expression of human homolog of achaete-scute complex (hASH1) mRNA in UIP lungs suggests that hASH1 could play roles in the regulation of PNEC.