Previous observations do not allow to draw firm conclusions on the effect of altitude sojourn on sleep, breathing and daytime performance in patients with the obstructive sleep apnea syndrome. Furthermore, the susceptibility of sleep apnea patients to high altitude related illness is not known. Therefore, the purpose is to study untreated patients with obstructive sleep apnea syndrome living at low altitude during a sojourn of a few days at moderate altitude in order to evaluate the physiologic effects of hypobaric hypoxia in these patients. We hypothesize that: 1. Sleep and nocturnal breathing disturbances in untreated OSA patients are more pronounced at moderate altitude compared to low altitude. 2. The increase in sleep related breathing disturbances at moderate altitude is due to an increase in central apnea/hypopnea