Posttraumatic stress disorder (PTSD) is a severely debilitating anxiety disorder. Over 80% of patients with PTSD also exhibit other psychiatric condition, such as bipolar disorder (BP) or major depression (MDD). Previously, it has been found that p11 mRNA expression was significantly changed in post mortem cortex of patients with PTSD and depression. We hypothesize that p11 mRNA levels in the peripheral blood cells will be a potential biomarker for PTSD with heterogeneity in terms of type of trauma, time since trauma and duration of illness. We examined the peripheral blood mononuclear cell (PBMC) P11 mRNA of patients with PTSD (n=13), major depressive disorder (MDD, n=16), bipolar disorder (BP, n=24), and schizophrenia (SCZ, n=12) or controls (n=14) using quantitative real-time PCR and the circulating levels of cortisol in blood plasma and saliva of PTSD using radioimmunoassay kit CORT-CT2. The Hamilton Rating Scale for Depression (HAMD) and Anxiety (HARS), the Chinese version of the Davidson Trauma Scale-Frequency (CDTS-F) and the Chinese version of the Davidson Trauma Scale-Severity (CDTS-S), and Impact of Event Scale-Revised (IES-R) were administered. We found that patients with PTSD had lower levels of p11 mRNA than control subjects, while those with MDD, BP and SCZ had significantly higher p11 levels than the controls. P11 mRNA levels were positively correlated with the scores of HAMD (r=0.62, p<0.05), CDTS-F (r=0.71, p<0.05) and CDTS-S (r=0.62, p<0.05), while they did not correlate with scores of HARS and IES-R. Basal levels of plasma and salivary cortisol of PTSD patients were not statistically different from those of controls. Our findings suggest that PBMC p11 mRNA expression levels may serve as a potential biomarker to distinguish PTSD from BP, MDD and SCZ.