At least three conditions contribute to COPD. (1) Chronic bronchitis (mucous hypersecretion) is an inflammatory condition in which CD8+ T-lymphocytes, neutrophils, and CD68+ monocytes/macrophages predominate. The condition is defined clinically by the presence of chronic cough and recurrent increases in bronchial secretions sufficient to cause expectoration. There is enlargement of mucus-secreting glands and goblet cell hyperplasia, which can occur in the absence of airflow limitation. (2) Adult chronic bronchiolitis (small or peripheral airways disease) is an inflammatory condition of small bronchi and bronchioli in which there are predominantly CD8+ and pigmented macrophages. The functional defect is difficult to detect clinically but may be recognized by sophisticated tests of small airway function. There is mucous metaplasia, enlargement of the mass of bronchiolar smooth muscle, and loss of alveolar attachments. (3) Emphysema is an inflammatory condition of the alveoli in which T-lymphocytes, neutrophils, and pigmented alveolar macrophages are involved, associated with the release of excessive amounts of elastases. It is defined anatomically by permanent, destructive enlargement of airspaces distal to terminal bronchioli without obvious fibrosis. In contrast, asthma is a clinical syndrome characterized by allergic inflammation of bronchi and bronchioli in which CD4+ (helper) T-lymphocytes and eosinophils predominate. There is increased production and release of interleukin (IL)-4 and IL-5, which is referred to as a Th2-type response. There is usually increased tracheobronchial responsiveness to a variety of stimuli, and the condition is usually manifest as variable airflow obstruction. While differences between COPD and asthma have been highlighted, new data are emerging that indicate there may also be similarities.