In 49 patients with obstructive airways disease and 15 normal subjects O2 and CO2 tension in arterial blood, vital capacity, maximal midexpiratory flow rate and forced expiratory volume were studied at 524, 1,600, and 2,844 m altitude. There was an acute decrease in O2 and CO2 tension and an increase of maximal midexpiratory flow rate. After an adaptation period of 10-12 days to 1,600 m in 70% of the patients an increase of the O2 tension and a decrease of the airways obstruction was observed. 30 % of the patients showed during this period a further decrease in the O2 tension without significant changes in airways obstruction