Eight patients with mild-to-moderate chronic obstructive pulmonary disease (COPD) and average resting Pao2 of 66 mm Hg were studied clinically and physiologically at sea level and after ascent to 1,920 m. At sea level the patients were symptomatic but not disabled. After ascent the patients had only mild symptoms of fatigue and insomnia, and one had severe headache during exercise on the first day. Funduscopic changes were not observed, nor did cardiac or pulmonary findings change. Resting sea level Pao2 dropped to 51.5 mm Hg within three hours of ascent, and the Paco2 fell from 37.8 to 33.9 mm Hg. Over the next three days, the Pao2 increased to 54.5 mm Hg as hyperventilation continued. At exercise, sea level Pao2 dropped from a mean value of 63 to 46.8 mm Hg at altitude. Pulse rates at rest or exercise did not change. Normal values for 2,3-diphosphoglycerate (2,3-DPG) did not change after ascent at 16 and 42 hours. We believe aircraft flight or travel to moderate altitudes for this type of COPD patient is safe. Preexisting hypoxemia resulting from disease may facilitate the adaptation of patients to severe hypoxia and may prevent symptoms similar to acute mountain sickness.