Introduction. High-altitude parachuting has two modalities HALO and HAHO, in both the skydiver needs oxygen supply to make the jump due to the hypoxic conditions associated with the altitude. Anticipating each subject’s response to hypobaric hypoxia (equivalent to altitude sickness) can serve to prevent fatal consequences.  Tolerance to hypoxia can be achieved with altitude simulators, subjecting the person to this stressful situation in a controlled manner in a normobaric environment. Objective: To determine the presence of paratroopers with poor tolerance to hypoxia in participants in a high-altitude skydiving course, analysing the associated factors. Method: Permission was obtained from the University’s Research Ethics Committee and the informed consent of the participants. 23 male paratroopers (30.6±6 years old) belonging to the Spanish Air and Space Force underwent a tolerance test for normobaric hypoxia (TTHN) consisting of breathing oxygen-depleted air (FiO₂ = 11%) equivalent to 5050 meters altitude for a maximum of 10 minutes. They were seated in front of an Ialtitude® simulator, the electrocardiogram (ECG) was monitored with a Nuubo® ambulatory device. Heart rate (HR) and peripheral oxygen saturation (SpO₂) are also measured using a pulse oximeter placed in the left ear. Participants held the device’s mask in their hands and breathed at their usual breathing rate. A monitor showed the evolution of HR and SpO₂. The safety criterion, and the interruption of the test, was established if the SpO₂ fell below 83%, noting the time elapsed from the beginning to the end of the test, either by reaching the maximum or by interrupting. The participants were classified into two groups: Complete Group (CG), which reached ten minutes; and Incomplete Group (IG) did not reach ten minutes. Previously, body composition was obtained by bioimpedance, and medical history and physical-sports activities and parachuting experience were collected. ANOVA was used to compare groups and Pearson’s coefficient to correlate variables. Results. The characteristics of the population were height 177.5±8.5 cm; total weight 81.6±9.3 kg; fat percentage 19.2±4.4%. Average weekly hours of physical exercise 7.8±3.7 h/s. 12 paratroopers (52.2%) completed the 10 minutes and 11 (47.8%) did not reach this time and formed the IG. From the CG, two skydivers are classified as having very good tolerance to hypoxia (SpO₂ at the end of the test >95%) and the rest (10) as having good tolerance. The mean time of those who did not complete the test was 270.1±101.7 seconds (range 120-250 sec). No changes were seen in the ECG. HR increased significantly in both groups (CG p=0.017; IG p<0.000). The data on the values of age, body composition, physical activity, history of respiratory pathology, smoking and experience as a skydiver do not show significant differences between the two groups. Conclusion. There are no clinical or sports variables that can predict low tolerance to hypoxia, which is close to 50% of this population. We believe that low tolerance can be a risk factor and needs to be detected and prevented.