Ten patients with known but clinically stable chronic obstructive pulmonary disease (COPD) were studied while breathing low-flow (1-2 L/min) supplemental oxygen by nasal cannula and by biflow nasal mask. Although the increase in inspired oxygen when breathing with the cannula has been documented, the nasal mask has not been tested by measurement of PaO2 change. The mean age of the group was 64.6 years, and their mean PaO2 was 54 +/- 10.5 torr while they were at rest and breathing room air. Patients were each given oxygen alternately with the mask and cannula, with no change in flowrate; time was allowed between the two for equilibration and return to baseline values. PaO2 values were compared by t test for nonindependent samples, which provided a t value of 1.893, with P = 0.09. Differences in measured arterial saturation (SaO2) and PaCO2 also proved to be insignificant when patients breathed via the two devices. The conclude that the nasal cannula and the biflow nasal mask produce comparable PaO2 in patients who may need low-flow supplemental oxygen at rest. The mask can be considered an alternative oxygen appliance if preferred by patients because of comfort.