Post-traumatic stress disorder (PTSD) frequently occurs in commorbidity with different mental disorders, including suicidal behaviour. Group of biological factors, including serotonergic system, HPA axis and some genetic factors, are being studied as potential markers, able to differentiate suicidal and non-suicidal behaviour across the group of PTSD patients. This study is examining statistical relation between platelet serotonine concentration and serum cortisole concentration, within the group of PTSD patients with and without attempted suicide, treated at "Sveti Ivan" Psychiatric Hospital in Zagreb. The hypothesis of this study is that periferal biochemical markers are different across the groups of PTSD patients with and without attempted suicide and the group of healthy controls. Our results have shown significantly lower platelet serotonine concentration in PTSD patients with and without suicide behaviour, compared to healthy controls. There are no statistically significant differences of the serum cortisole concentration across observed groups. Our results correspond with those reported by other authors in this area of research, suggesting that platelet serotonine level might be used as potential periferal marker to detect risk of suicidal behaviour in PTSD patients.