The assessment of an individual's degree of acclimatization to altitude is difficult. This is particularly applicable to military operations that have to be performed at altitude. This study describes a new and simple test that allows for the determination of an individual's risk for high-altitude illness at higher altitudes. The prediction is based on the lowest oxygen saturation (SaO2) found during an uphill run at high altitude (11,060 ft [3,371 m]), combined with the time needed to complete the run. The test results were compared against the severity of high-altitude symptomatology on the summit of Mont Blanc (15,762 ft [4,808 m]). The main outcome was the significant correlation between time as well as SaO2 and the severity of high-altitude symptomatology on the summit of Mont Blanc. The newly developed performance test allows, at a "safe" altitude, the prediction of an individual's risk of developing high altitude illness if they continue to ascend. It allows the determination of the best acclimatized subjects within a group, for example, before a military mission at greater altitude.