Hypoxemic patients with chronic obstructive pulmonary disease (COPD) are at risk of carbon dioxide (CO(2)) retention during oxygen therapy and hypercapnia in COPD is associated with an ominous prognosis. Rebreathing with oxygen mask is possible in practice and possibly affects CO(2) retention due to an increased inspired fraction of CO(2). Its effects on arterial partial pressure of CO(2) during oxygen supply have, to the best of our knowledge, never been studied. We measured the inspired fraction of CO(2) in eighteen non-hypoxemic stable COPD patients with a capnograph during a 5 min trial with two different modes of oxygen supply (oxygen mask without reservoir bag and nasal prongs, respectively at a flow of 10 l/min and 2l/min). We found no significant increase in inspiratory CO(2) concentration. These findings suggest that inspired fraction of CO(2) does not increase markedly during controlled oxygen therapy.