Progressive loss of body-mass during high-altitude sojourns may be largely caused by decreased food intake, possibly due to hypobaric hypoxia. Therefore we assessed the effect of long-term hypobaric hypoxia per se on appetite and food intake. Eight men were exposed to a 31-day simulated stay at several altitudes up to the peak of Mt. Everest (8,848 m). Palatable food was provided ad libitum, and stresses such as cold exposure and exercise were avoided. At each altitude, body-mass, energy-expenditure (EE), energy and macronutrient intake, attitude toward eating and appetite profiles during and between meals were assessed by using questionnaires. Body-mass reduction of an average of 5 +/- 2 kg was mainly due to a reduction in energy intake of 4.2 +/- 2 MJ/day (P < 0.01). EE had slightly decreased. At 5,000- and 6,000-m altitudes, subjects had hardly any acute mountain sickness symptoms and meal size reductions (P < 0.01) were related to a more rapid increase in satiety (P < 0.01). Meal frequency was increased from 4 +/- 1 to 7 +/- 1 eating occasions per day (P < 0. 01). At 7,000 m, when acute mountain sickness symptoms were present, uncoupling between hunger and desire to eat occurred and prevented a food intake necessary to meet energy balance requirements. On recovery, body mass was restored up to 63%; this suggests physiological fluid retention with the return to sea level. Exposure to hypobaric hypoxia per se was associated with a change in the attitude toward eating and with a decreased appetite and food intake, resulting in body-mass loss.