If one includes all types of chronic generalized airways obstruction under the heading of "COPD," diagnosis of this condition requires only the demonstration of an obstructive ventilatory impairment on spirometric testing that persists despite maximum medical therapy. However, as generally used, the term COPD implies that upper airways obstruction and "specific" lung diseases that can produce an obstructive type of physiologic abnormality have been excluded. Examples of these exclusions include silicosis, sarcoidosis, and even advanced tuberculous disease. It is more difficult to determine the type of disease that is causing the chronic airways obstruction in patients with COPD as defined above. A severe and persistent form of asthma, sometimes called "chronic asthmatic bronchitis," can mimic the typical emphysematous form of COPD that is characteristic of heavy cigarette smokers. Since these types of chronic airflow obstruction differ in regard to their clinical courses, prognoses, and treatments, their distinction is clinically important. One should not be discouraged by the fact that some patients appear to have a mixed type of disorder. Features that help differentiate the various forms of chronic airways obstruction are described in this report, and recommendations are offered to help guide the practitioner in the workup indicated for patients thought to have any type of chronic airways obstruction. It is also emphasized that patients vary markedly in regard to the relative importance of readily reversible bronchospasm, airways inflammation, and mucus hypersecretion in producing their disability. Assessment of these factors is critical in determining clinical management.