Objective : The objective of this study was to examine the relationship between plasma serotonin concentration and posttraumatic stress disorder (PTSD) symptoms in chronic PTSD patients who have been taking medication. Methods : Plasma serotonin level of 14 PTSD patients and a control group of 28 Vietnam War veterans was measured by HPLC (high performance liquid chromatography). The Combat Exposure Scale (CES), Mississippi Scale for Combat-Related Posttraumatic Stress Disorder (M-PTSD), Clinician Administered PTSD Scale (CAPS), Hamilton Rating Scale for Depression (HRSD), and Hamiltion Anxiety Scale (HAS) were used to evaluate PTSD symptom severity. Results : Serotonin level was significantly higher in the PTSD group than in the control group (p=0.036, p=0.006, respectively). M-PTSD (p<0.001), CAPS (p<0.001), HRSD (p<0.001), and HAS (p<0.001) scale scores were significantly higher in the PTSD group than in the control group; however, the CES score failed to show a significant improvement (p=0.964). There were no significant differences between plasma serotonin and PTSD symptoms. Conclusion : In chronic PTSD patients who have been taking medications, we can not predict treatment effect and symptom severity by measuring only plasma serotonin levels. PTSD is a complicated disorder which may likely be related to a variety of neurotransmitter systems. Therefore, further research which investigate relationships with norepinephrine, dopamine, and other neurotransmitters as well as serotonin is needed to improve the treatment of PTSD.