Limitation of physical activity occupies a central role in the symptom complex of patients with chronic obstructive pulmonary disease (COPD), and improvement in exercise capacity is a key outcome of response to COPD therapy. Maximum exercise capacity testing facilitates assessment of physiologic mechanisms of exercise and allows quantitation of the degree of limitation. This manuscript utilizes published data from the National Emphysema Treatment Trial to investigate the minimal clinically important difference (MCID) in maximum exercise capacity in patients with severe emphysema. Distribution- and opinion-based methods were used to estimate MCID. Expert clinician opinion yielded a value of 10 Watts as the MCID for change in maximum exercise capacity. Baseline standard deviation and error data yielded a one-half standard deviation-based estimate of 10.5 Watts and a standard error-based estimate of 4.2 Watts. In subjects randomized to medical therapy, the mean (+/-SD) 24-month change in maximum exercise capacity following medical therapy was -9.2 +/- 1.2 Watts, whereas among those randomized to lung volume reduction surgery, mean 24-month change in maximum exercise capacity was 1.7 +/- 17.7 Watts, with a mean difference between the groups of 10.9 Watts. The observed difference in maximum exercise capacity after 24 months between subjects randomized to medical versus surgical therapy conforms to both opinion- and distribution-based estimates of MCID. Further investigation is needed to develop and validate estimates of MCID for maximum exercise capacity and other key clinical outcomes in COPD.