Diverticulosis of the bronchial wall was found in patients not only with, but also without, chronic obstructive lung disease; it appeared to start as submicroscopic depressions and dilatations of the ducts of the bronchial gland on the mucosal surface. Multiple depressions and dilatations fused to form a diverticulum which herniated between and through the smooth-muscle cellular bundles. Rupture of the latter resulted in large diverticula. Cough and a weakened bronchial wall, from whatever causes, likely lead to bronchial diverticulosis. Exaggerated but unequal formations of bronchial diverticula at the sites of dichotomy suggest either that the effect of cough could be different between segments or subsegments, or that there are local differences in connective-tissue atrophy, inflammation, and structural defects. Mucous plugs, macrophages, red blood cells, inhaled particles, and probably carcinogens are accumulated at the bronchial diverticula, which apparently interfere with airway cleansing and also cause continuous local irritation. The relationship between bronchial diverticulosis and small-airway disease or lung cancer needs further clarification.