Studies of mechanics and morphometry were performed in postmortem examination of human lungs. Left lungs were obtained from male victims of sudden non hospital death on whom autopsies were performed in the coroner's office. The results reported are from the first 26 lungs studied. Bronchial mucous gland hyperplasia is one of the pathologic abnormalities described in chronic bronchitis. When mucous glands occupy 15% or more of the bronchial wall, they are considered hyperplastic. The percentage of mucous glands in the bronchial wall exceeded this value in 6 of the 26 lungs studied (range 16.1 to 24.1%). The size of the central airways, as reflected by the mean segmental bronchial diameter does not change significantly beyond the age of 20 yr. That is, mean bronchiole diameter increases through the third decade of life; thereafter it decreases at a variable but highly significant rate. This previously unreported finding may have important implications in concepts of the pathogenesis of chronic destructive pulmonary disease (COPD). However, mean bronchiole diameter in the 5 lungs with mild emphysema is not significantly different from age matched nonemphysematous lungs. The usual signs of tissue injury, such as inflammation and fibrosis, are inconstant in the narrowed bronchioles of the older lungs. The subtle decrease in diameter can only be appreciated by measurement. Furthermore, in experimental animals it is extremely difficult to induce significant bronchiolar stenosis despite prolonged exposure to high concentrations of a toxic gas such as nitrogen dioxide. These findings in animals, in addition to observations in human lungs in which many narrowed bronchioles are present without pathologic evidence of injury and repair, suggest that the decrease in diameter is in part related to variation in development or aging.