The neurobiological basis of posttraumatic stress disorder (PTSD) is believed to involve alterations in different neurotransmitter systems, and recent studies elucidated the role of serotonin (5-hydroxytryptamine, 5-HT) in PTSD. The data on the role of 5-HT have been obtained using blood platelets as a peripheral model for central serotonergic neurons. The reports suggested that platelet 5-HT concentration and monoamine oxidase (MAO) activity might serve as biological, even trait, markers for particular mental disturbances. Since the data on the peripheral serotonergic markers in PTSD subjects are controversial, the aim of the study was to determine platelet 5-HT concentration and platelet MAO activity in war veterans with PTSD, war veterans who did not develop PTSD, and in war veterans who were prisoners of war and developed PTSD. Platelet 5-HT concentration and MAO activity did not differ significantly between war veterans with or without PTSD, and prisoners of war with PTSD. Clinician-Administered PTSD Scale (CAPS) scores did not differ between war veterans with PTSD and prisoners of war, but Montgomery-Asberg Depression Rating Scale (MADRS) scores were significantly higher in prisoners of war who developed PTSD than in war veterans with PTSD. There was no significant correlation between platelet 5-HT concentration or platelet MAO activity and CAPS or MADRS scores within these groups. Platelet 5-HT concentration was slightly higher and platelet MAO activity slightly lower in prisoners of war with PTSD, than in all other groups. These findings suggest that platelet 5-HT concentration and platelet MAO activity are not altered in three drug-free groups--war veterans who did or did not develop PTSD, or in prisoners of war with PTSD--and that these platelet serotonergic markers are not associated with symptoms of PTSD or comorbid depression.