We monitored arterial oxygen saturation (SaO2) in 10 patients with severe chronic obstructive lung disease during 24 h of breathing room air followed by 24 h of breathing two litres of oxygen per minute. Three subjects without chronic obstructive lung disease were monitored while breathing room air. Greatest declines in SaO2 occurred during sleep, with intermittent decreases as great as 44% saturation (range, 12% to 44% saturation. Baseline SaO2 was significantly higher while patients breathed low-flow oxygen (94% versus 86% saturated), and declines in SaO2 during sleep were less noteworthy (1% to 27% saturation). Subjects without chronic obstructive lung disease showed declines in SaO2 of much lesser magnitude (3% to 11% saturation) with sleep. These declines were not primarily due to alveolar hypoventilation. Our data indicate that there are patients with chronic airways obstruction who suffer profound intermittent desaturation at night that can readily be relieved with low-flow oxygen administration.