Nine patients with chronic obstructive airways disease performed a 6 min self-paced walk (breathing air) on a treadmill and then identical (but operator-controlled) treadmill walks breathing either air or supplemental oxygen sufficient to just prevent arterial oxygen desaturation during the exercise. During the exercises, ventilation was recorded and patients recorded their sensation of breathlessness on a visual analogue scale (VAS) every 30 s. Breathing supplemental oxygen produced a small fall in mean exercise ventilation and a large and consistent reduction in mean exercise breathlessness. In seven patients the VAS scores were higher on air than with supplemental oxygen, at similar levels of ventilation. An analysis of covariance, to control for reduction in ventilation, showed a decrease in mean breathlessness when breathing supplemental oxygen, significant at the 8% level. The reduction in breathlessness produced by preventing exercise desaturation cannot be explained by the decrease in ventilation. This suggests that hypoxia may be a stimulus for breathlessness. The mechanism is unknown.