circles. Subsets of patients lying within the rectangle have obstruction of their airways. Patients with asthma, subset 9, are defined as having completely reversible airways obstruction and lie entirely within the rectangle; their diagnosis is unequivocal. Patients in subsets 6 and 7 have reversible airways obstruction with chronic productive cough or emphysema, respectively. Patients in subset 8 have features of all three disorders. It may be difficult to be certain whether patients in subsets 6 and 8 indeed have asthma or whether they have developed bronchial hyperreactivity as a complication of chronic bronchitis or emphysema; the history helps. Patients in subset 3 have chronic productive cough with airways obstruction but no emphysema; it is not known how large this subset is, since data from epidemiologic studies using the computer tomography scan, the most sensitive in vivo imaging technique for diagnosing or excluding emphysema, are not available. It is much easier to identify in the chest radiography patients with emphysema who do not have chronic bronchitis (subset 4). Most patients in subsets 1 and 2 do not have airways obstruction as determined by the FEV1 (forced expiratory volume exhaled in 1 second) but have clinical or radiographic features of chronic bronchitis or emphysema, respectively. Because COPD, when defined as a process, does not have airways obstruction as a defining characteristic, and because pure asthma is not included in the term COPD, patient subsets 1–8 are included within the area outlined by the shaded band that denotes COPD (Snider, 1988).