Bronchiolar inflammation and fibrosis associated with smoking. A morphologic cross-sectional population analysis. The lungs of 42 smokers and 13 nonsmoking males of various ages who died suddenly and unexpectedly were examined grossly using Gough-Wentworth whole-lung sections and by microscopic planimetry to assess the severity and prevalence of emphysema. The bronchioles in representative histologic sections were evaluated for inflammation and epithelial metaplasia as well as for fibrosis and muscular hypertrophy. Postmortem interviews with next of kin established a history of cigarette smoking and excluded possible occupational exposures to toxic or particulate inhalants. Emphysematous changes were not prominent in members of the study group, but they tended to be more severe in smokers (p = 0.059) and increased in severity with age (p less than 0.001). Inflammatory changes (so-called smoker's bronchiolitis) were evident in smokers of all ages, although they were significantly less prominent in the lungs of older smokers. On the other hand, respiratory and membranous bronchiolar wall fibrosis was increasingly evident in older smokers (p less than 0.05). Muscular hypertrophy in the bronchiolar walls was significantly greater in smokers, but a change with age was not observed. These findings strongly suggest that bronchiolar fibrosis is associated with chronic cigarette use. These lesions occur independently of emphysema and may account for some of the subtle physiologic alterations observed in smokers. 