Classically, asthma and chronic obstructive pulmonary disease present distinct clinical, physiologic and pathologic features.However, not infrequently, patients may present with overlapping clinical symptoms and physiological abnormalities: patients with severe asthma may present with fixed airway obstruction and patients with chronic obstructive pulmonary disease may have hyperresponsiveness and eosinophilia. At the pathological level, inflammatory and structural similarities also occur and may be related to the phenotypic overlaps. On the basis of such similarities in the inflammatory process, an argument can be made for a similar approach to treatment. However, both the response to treatment and the natural histories of the two diseases are quite different. While they share some common features, they can be distinguished in several important aspects.