In the present study, we investigated the effect of a long-term mountain expedition on glucose tolerance and insulin action. Twelve registered mountaineers ages 31 years (SD = 1.1) participated in a 25-day expedition at a 2,200-3,800-m altitude with an average duration of 8 hr per day. Arterial oxygen saturation (SaO[subscript 2]) was substantially reduced during hiking. Glucose tolerance and insulin responses were measured prior to and twice during the expedition period. Maximal oxygen consumption increased from 43.0 [plus or minus] 2.7 to 49.1 [plus or minus] 2.2 mL/kg/min. Percentage of body fat decreased from 19.4 [plus or minus] 6.8% to 16.9 [plus or minus] 5.9%. The area under the curves for insulin and glucose during the oral glucose tolerance test were also reduced in Days 3 and 25. The present study demonstrated that altitude hiking activity is an effective lifestyle intervention to improve insulin action. (Contains 4 tables and 2 figures.)