Existing evidence and the diathesis-stress model hypothesis suggest that stress as an environmental factor may trigger the onset of psychiatric disorders, such as psychosis spectrum disorders, mood disorders, anxiety disorders, in people with an underlying vulnerability. The purpose of this study was to determine the period of time during military service at which symptomatology of clinical significance is more often developed, considering that stress of service and adaptation to its requirements is common to all army recruits. A retrospective file study for the years 2017-2018 was conducted in order to identify male soldiers who were hospitalized in the psychiatric clinic of 414 Athens Military hospital and diagnosed as F20-29, F30-39 and F40-48 according to ICD-10. The number of hospitalizations per clinical diagnosis and the time of onset relatively to the month of military service were examined. A total of 139 cases were screened, 119 of which had disorders falling into the diagnostic categories F20-29, F30-39 and F40-48. 53% of total hospitalizations took place within the first two months of a nine-month military service. It was found that the risk of disorder onset within the first two months of military service was statistically higher (OR=0.210, p=0.001) for a schizophrenic spectrum disorder (F20-29) compared to F30-39 and F40-48 disorders. After adjusting for potential confounders, such as heritability for psychiatric disorders, urbanicity, history of substance use or age, the risk for hospitalization within the first two months continued to be significantly predicted by F20-29 disorder (OR=0.255, p=0.022) compared to other diagnoses. Individuals vulnerable in developing a psychotic disorder appear to have lower stress tolerance and may manifest an earlier disease onset, when exposed to the stress of military service compared to subjects predisposed to develop mood or anxiety disorders. Stress during military service is of major importance for the emergence of the whole psychopathology spectrum, particularly in vulnerable individuals. The development of prevention and early intervention strategies is considered to be of particular importance to conscripts. The findings of the present study are in agreement with the diathesis-stress model and moreover suggest that people who subsequently develop a psychotic spectrum disorder have greater vulnerability to stress exposure.