The extent of air-space destruction caused by emphysema is very variable in severe chronic obstructive pulmonary disease (COPD), constituting one of the most obvious differences between COPD and asthma. Differences in the static deflation pressure-volume curve between COPD and asthma can easily be shown, but it has been surprisingly difficult to find distinctive mechanical features of impaired airway function caused by air-space destruction. This may be because in mild airway obstruction related to smoking--particularly in younger subjects--emphysema may be absent, and the predominant site of airway narrowing in the smallest bronchi and respiratory bronchioles may be the same as that found in asthma in remission. In more severe obstruction caused by COPD there is almost always very severe intrinsic disease of the airways and this may so dominate the functional abnormality that it is difficult to detect any additional change because of airspace destruction. Overall, few studies have set out to detect specific effects of parenchymal destruction on airway function.