Patients with cystic fibrosis (CF) suffer from hypoxaemia even under normobaric conditions and the reduction of inspiratory PO2 (O2 partial pressure) during air travel corresponding to an altitude of 1,800-2,450 m might be a problem for these patients. Ten CF patients and 27 healthy control subjects were investigated in a chamber where the ambient pressure was reduced to that found at 2,000 and 3,000 m. The respiratory function was reduced in the CF patients with a vital capacity of 3.1 (0.3) l [vs 4.9 (0.2) l in controls; mean (SEM)] and a forced expiratory 1-s volume of 2.1 (0.3) l [vs 4.3 (0.20 l in controls], unrelated to the reduction in ambient pressure. Mean arterial PO2 decreased from 75 (4) mmHg [85 (1) mmHg in controls, P<0.01] at sea level to 58 (3) mmHg at 580 mmHg and to 46 (1) mmHg [58 (1) mmHg and 49 (2) mmHg in controls, n.s.] at 513 mmHg ambient pressure. These results indicate that during air travel with a cabin pressure that corresponds to an altitude of 2,500 m, the arterial PO2 of CF patients is likely to remain above the accepted critical value of 50 mmHg. However, a further reduction of the pressure to that found at 3,000 m altitude may lead to severe hypoxia in patients with moderate airway obstruction.