The structural and physiologic findings in asthma and COPD appear, on average, and in the extremes of presentation, to be easily distinguished. A closer inspection of the literature reveals that significant overlap exists in individual patients with respect to airway wall thickening and low-attenuation parenchymal regions on CT scans, and in reversibility, airway hyperresponsiveness, lung diffusion, resting and dynamic hyperinflation, lung elastic recoil, exercise response, and a "pharmaceutical volume reduction" effect following therapy with bronchodilators. In particular, the subgroup of COPD patients having an airway-dominant phenotype becomes indistinguishable from asthmatic subjects with reversible disease that evolves into an incompletely reversible pattern.