The aim of the study was to investigate hypoxaemia in chronic obstructive pulmonary disease patients during a commercial flight. The effect of a commercial flight, lasting 5 h 40 min, on arterial blood gas levels and symptoms in 18 chronic obstructive pulmonary disease patients with a pre-flight percutaneous oxygen saturation of 94% and self-reported ability to walk 50 m without severe dyspnoea was studied. The arterial oxygen tension (Pa,O2) decreased from sea level to cruising altitude (10.3+/-1.2 versus 8.6+/-0.8 kPa), but, thereafter, except for one patient, remained stable throughout the flight. During light exercise, however, there was further desaturation (percutaneous oxygen saturation 90+/-4 versus 87+/-4%). After 4 h, a decrease in arterial carbon dioxide tension (5.0+/-0.4 versus 4.8+/-0.4 kPa) and an increase in cardiac frequency (87+/-13 versus 95+/-13 beats x min(-1)) were observed. A pre-flight Pa,O2 of >9.3 kPa did not secure an acceptable in-flight Pa,O2. Aerobic capacity showed the strongest correlation with in-flight Pa,O2. In conclusion, following an initial decrease in arterial oxygen tension, chronic obstructive pulmonary disease patients in a stable state of their disease seem to maintain a stable arterial oxygen tension throughout a flight of intermediate duration, except when walking along the aisle. However, a decrease in arterial carbon dioxide tension, indicating compensatory hyperventilation, could imply a risk of respiratory fatigue during longer flights.