Background: Intermittent hypoxia has been suggested to increase exercis e toler ance by enhancing stress resistance and improving oxygen delivery. Because the i mprovement of exercise tolerance reduces mortality in the elderly with and witho ut coronary artery disease intermittent hypoxia might be a valuable preventive a nd therapeutic tool. However, controlled studies are lacking. Methods and result s: Sixteen males (50-70 years, 8 with and 8 without prior myocardial infarction ) were randomly assigned in a double-blind fashion to receive 15 sessions of pa ssive intermittent hypoxia (hypoxia group) or normoxia (control group) within 3 weeks. For the hypoxia group each session consisted of three to five hypoxic (14 -10%oxygen) periods (3-5 min) with 3-min normoxic intervals. Controls inhale d only normoxic air in the same way. Exercise tests were performed before and af ter the 3-week breathing program. After 3 weeks of intermittent hypoxia peak ox ygen consumption had increased compared to normoxic conditions (+6.2%vs.-3%, p< 0.001). This improvement was closely related to the enhanced arterial oxygen content after hypoxia (r=0.9,p< 0.001). Both higher haemoglobin concentration a nd less arterial oxygen desaturation during exercise contributed to the increase in arterial oxygen content. During sub-maximal exercise (cycling at 1 W/kg) he art rate, systolic blood pressure, blood lactate concentration, and the rating o f perceived exertion were diminished after intermittent hypoxia compared to cont rol conditions (all p < 0.05). Changes in responses to exercise after intermitte nt hypoxia were similar in subjects with and without prior myocardial infarction . Conclusions: Three weeks of passive short-term intermittent hypoxic exposures increased aerobic capacity and exercise tolerance in elderly men with and witho ut coronary artery disease. Background: Intermittent hypoxia has been suggested to increase exercis e toler ance by enhancing stress resistance and improving oxygen delivery. Because the i mprovement of exercise tolerance reduces mortality in the elderly with and with ut coronary artery intermittent hypoxia might be a valuable preventive a nd However, controlled studies are lacking. Methods and results s: Sixteen males (50-70 years, 8 with and 8 without prior myocardial infarction) were randomly assigned in a double-blind fashion to receive 15 sessions of pausing (hypoxia group) or normoxia (control group) within 3 weeks. For the hypoxia group each session consisted of three to five hypoxic (14 -10% oxygen) periods (3-5 min) with 3-min normoxic intervals. only normoxic air in the same way. Exercise tests were performed before and af ter the 3-week breathing program. After 3 weeks of intermittent hypoxia peak ox ygen consumption had increased compar The improvement was closely related to the enhanced arterial oxygen content after hypoxia (r = 0.9, p <0.001). Both higher hemoglobin concentration a nd less arterial (+0.2% vs.-3%, p <0.001) During sub-maximal exercise (cycling at 1 W / kg) he art rate, systolic blood pressure, blood lactate concentration, and the rating of perceived exemple were diminished after intermittent hypoxia compared Changes in responses to exercise after intermittent nt hypoxia were similar in subjects with and without prior myocardial infarction. Conclusions: Three weeks of passive short-term intermittent hypoxic exposures increased aerobic capacity and exercise tolerance in elderly men with and witho ut coronary artery disease.