Classification of chronic airflow obstruction may be based on the site of the obstructing lesions. It is seldom that only one type of lesion is present, but one may often dominate. In chronic bronchitis, the major disease of large airways, chronic mucus hypersecretion, is reflected by an increase in size of bronchial mucous glands. This may be a factor in airway narrowing, especially with coexisting edema of the airway wall. Excess intralumenal mucus compounds the obstruction. Increased airways reactivity is present in 15 to 70 percent of patients with chronic airflow obstruction. Increased airway muscle and cartilage atrophy are features of chronic bronchitis, but the association of increased muscle with increased airway reactivity is poor. Inflammation of the small airways (bronchiolitis) is a significant complication for cigarette smokers and is an important cause of mild chronic airflow obstruction. Goblet cell metaplasia is a reflection of chronic small airways inflammation and, together with intralumenal mucus, is an important feature. Permanent narrowing of the small airways presumably results from inflammation with consequent fibrosis, while functional narrowing results from release of mediators of inflammation. Increased muscle mass is present in some cases. Distortion and irregularity of small airways related to emphysema are major factors in severe obstruction. Lesser degrees of emphysema may be associated with a diminished number of alveolar attachments and mild chronic airflow obstruction. Emphysema, the dominant lesion in patients with severe chronic airflow obstruction, results from parenchymal lesions. Centrilobular emphysema, in which the respiratory bronchioles are selectively or dominantly involved, is the most common form.(ABSTRACT TRUNCATED AT 250 WORDS)