Oxygen therapy improves submaximal exercise tolerance in hypoxemic patients with chronic obstructive pulmonary disease (COPD). This study compared the standard nasal cannula, reservoir nasal cannula, and a demand flow device in 15 male hypoxemic patients with COPD. On six separate occasions each subject used, in a randomized order, all three systems while completing incremental cycle ergometry and a test circuit composed of tasks that simulate activities of daily living. Oxygen flow required during exercise was 1.8 +/- 0.9 and 2.8 +/- 0.7 L/min for reservoir nasal cannula and standard nasal cannula use, respectively (p < 0.0001). The effect of the three oxygen delivery systems on oxygen saturation (Spo2) during the last 30 s of exercise varied with type of activity. Only during demand flow device use while undressing and dressing was the subjects' Spo2 (90 +/- 3%) significantly lower (p = 0.019). There was a trend toward lower Spo2 with the demand flow device (p = 0.103) during arm work above shoulder level. Although not statistically significant, reservoir nasal cannula use resulted in consistently lower tidal volume and minute ventilation during test circuit activities. Exercise tolerance was not significantly different between the three oxygen delivery systems.