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Compulsory basic military training is characterized not only by being challenging but also by being stressful. Assuming a high frequency of perceived stress events as a negative outcome of training, this article provides evidence on how the perceived frequency of stressful situations is affected by three types of factors: (i) biological stress response variables measured by hair steroid hormone levels, (ii) personality traits measured using the Big Five personality test, and (iii) group cohesion measures in military squads. A total of 112 conscripts in 11 squads participated in the research at the beginning (T1), in the middle (T2), and at the end (T3) of compulsory basic military training. Hair steroid hormone levels (cortisol, cortisone, and dehydroepiandrosterone (DHEA)) levels were measured by liquid mass spectrometry; other data were collected using self-report questionnaires. The results of the Poisson regression analysis indicated that hair steroid hormone cortisol had a statistically significantly impact and could increase the perceived frequency of stressful situations by up to 1.317 (e(0.275), T2) times. The concentrations of other hormones (cortisone = 1.157, e(0.146), T3 and DHEA = 1.020, e(0.020), T3) also had a statistically significant effect. Other factors had a decreasing effect on the frequency. Extraversion was significant with an effect of 0.907 (e(-0.098), T2) and 0.847 (e(-0.166), T3), while task cohesion had an effect of 0.946 (e(-0.056)) and norm cohesion of 0.954 (e(-0.047)). The research indicates that the three groups of factors affect the perceived frequency of stressful situations during compulsory basic military training, but their impacts are considerably different.