The diseases included are those commonly called chronic bronchitis, bronchiolitis, bronchiectasis and asthma. The definitions most commonly used are those called for and applied in epidemiological studies. In this symposium, it is the role of inflammation in these various diseases that will be explored. The term bronchiectasis describes bronchial or bronchiolar distortion and scarring. It is well described by the terms bronchitis obliterans and bronchiolitis obliterans. In the 'wet' form of the disease, chronic sputum production is a feature. The factors that cause mucus hypersecretion in chronic bronchitis probably operate. Chronic bronchitis is characterized by mucus hypersecretion. Recent studies reveal that change in the nature of secretion is an early sign: new constituents appear before the amount is appreciably increased. Bronchial aspirate from normal non-smoking human volunteers, as judged by density gradient ultracentrifugation, contained no typical epithelial glycoprotein but did contain a glycoconjugate of higher buoyant density that included sugars characteristic of proteoglycans. In normal volunteers who had smoked, albeit mildly, macromolecular yield was not increased; however, a glycoconjugate of buoyant density typical of epithelial glycoprotein but with sugars characteristic of both proteoglycans and glycoprotein was identified. In simple, chronic bronchitis a typical epithelial glycoprotein can be identified. Study of the canine model of chronic bronchitis supports these findings. In organ culture, human and canine airways do not secrete the typical epithelial glycoprotein under baseline conditions but they do when stimulated, as by cholinergic agents. Analysis of lipids also yields a similar picture in human disease and in the canine model. Cholesterol is typical of normal mucus. When secretion is obviously increased, glycolipid is also present.