The variability in sensitivity to acute mountain sickness among individuals is a phenomenon well known to physicians and high altitude alpinists. The measurement of cardiac and respiratory responses to hypoxia (FIO2 = 0.115) at rest and during exercise (50% VO2max) allows the detection of those subjects who are more liable to suffer from high altitude diseases. In a retrospective study performed on 288 subjects evaluated with a hypoxic test during a Mountain medicine consultation, we found that the most clinically susceptible subjects had at least one abnormal response to the hypoxic tests, especially during exercise. The observation of one or several abnormal values in cardiac or respiratory responses to hypoxia leads us to advise a modification in the alpine or trekking objective, an increase in the acclimatization time and/or prevention by acetazolamide.