Exposure to high altitude induces physiological or pathological modifications that are not always clearly attributable to a specific environmental factor: hypoxia, cold, stress, inadequate food. The principal goal of hypobaric chamber studies is to determine the specific effect of hypoxia. Eight male volonteers ( « altinauts » ), aged 23 to 37 were selected. They were first pre-acclimatized in the Vallot Observatory (4,350m) before entering the chamber. The chamber was progressively (30 days) decompressed down to 253 mmHg barometric pressure, with a recovery period of 3 days at 5,000m in the middle of the decompression period. Eighteen protocoles were organized by 14 european teams, exploring the limiting factors of physical and psychological performance, and pathophysiology of acute mountain sickness (AMS). Ali subjects reached 8,000m and 7 of them reached the simulated altitude of 8,848m. Three altinauts complained with transient neurological symptoms which resigned rapidly with reoxygenation. Body weight decreased by 5.4 kg through a negative calorie balance. Four days only after the return to sea-level, subjects had recovered 3.4 kg, i.e. 63% of the total loss. At 8,848m, mean SaO2 was 68±3 %, PaO2 34 ± 4 mmHg, PCO2 12 ± 2 mmHg, pH 7,57 ± 0.02. Hemoglobin concentration increased from 14.8 ± 1.4 to 18.4 ± 1.5 g/dl at 8000m and recovered within 4 days at sea-level. AMS score increased rapidly at 6000m and was maximal at 7000m, especially for sleep. AMS was related to alteration in color vision and elevation of body temperatue. VO2max decreased by 59% at 7000m. The purpose of this paper is to give a general description of the study and the time course of main clinical and physiological parameters. The altinauts reached the "summit" (for some of them three consecutive times) in better physiological conditions than it would have been possible in the mountains, probably because acclimatization and other environmental factors such as cold and nutrition were controlled.