Because persons with chronic obstructive pulmonary disease and mild hypoxemia may develop severe hypoxemia during commercial airline flights, we measured arterial blood gas pressures in 13 such patients during a flight in an unpressurized airplane (cabin pressures typical of commercial air travel). At 1650 m, mean arterial Po2 decreased from 68.2 +/- 8.5 (SD) mm Hg to 51 +/- 9.1 mm Hg, and mean arterial Pco2, from 40.9 +/- 0.9 to 37.1 +/- 6.4 mm Hg. At 2250 m, mean arterial Po2 and Pco2 were 44.7 +/- 8.7 and 36.5 +/- 5.8 mm Hg, respectively. No symptoms attributable to hypoxemia occurred. Arterial Po2 measured in patients while breathing room air several weeks before the flight did not correlate with that measured at 1650 m, but arterial Po2 measured less than 2 hours before the flight in room air or a 17.2% oxygen mixture did. Whether a patient will need supplemental oxygen to maintain arterial Po2 above a given value can be predicted from arterial blood samples taken while the patient is breathing a hypoxic gas mixture or room air within 2 hours of the flight.