As illustrated by two cases, a fall in alveolar oxygen pressure at high altitude, such as during transportation by plane, can cause hypoxic pulmonary failure, especially in patients with already impaired pulmonary or circulatory functions. Arterial oxygen saturation was recorded by pulse oximetry in 14 healthy volunteers and eight patients during air transportation. Oxygen saturation decreased with reduced cabin pressure. Decreased saturation for each 100 mm Hg reduction in cabin pressure was markedly greater in the patients than the healthy volunteers. Pulmonary and circulatory status should be assessed before air transport of patients and if necessary departure delayed. Oxygen ought to be added to inspired air with appropriate supervision in all acutely ill patients. Indications for endotracheal intubation before flight should be generously defined. Continuous pulse oximetry is noninvasive and highly informative. It is urgently recommended that air ambulances be equipped with them.