Posttraumatic stress disorder (PTSD) is a severe psychiatric illness associated with disturbances in diverse neurobiological systems. The evaluation of a variety of biomarkers might facilitate a goal of modern medicine, a proper treatment for an individual patient at a given stage of disease. This is especially important in PTSD, a disorder with a complex clinical picture, diverse symptoms, and frequent comorbidities. Biological markers (platelet serotonin, platelet monoamine oxidase, plasma lipid levels, plasma dopamine beta hydroxylase, plasma cortisol and serum levels of thyroid hormones) were determined, and clinical symptoms were evaluated, in 93 male war veterans with chronic combat related PTSD, using the Clinician Administrated PTSD Scale, Positive and Negative Syndrome Scale, and the Hamilton Rating scales for Depression and Anxiety. Platelet serotonin concentration and plasma dopamine beta hydroxylase activity were similar in PTSD subjects and healthy controls. Platelet monoamine oxidase activity, and plasma/serum levels of total and free triiodothyronine and cortisol were increased in war veterans with PTSD compared to controls, indicating that these biomarkers might be used as the trait markers in PTSD. Since a great proportion of our war veterans with chronic combat related PTSD had a severe form of PTSD, complicated with the presence of psychotic or depressive symptoms, further studies are underway to elucidate the association between biological markers and particular symptoms occurring in PTSD.