Trauma survivors show marked differences in the severity and persistence of post-traumatic stress disorder (PTSD) symptoms. Early symptoms subside in most, but persist as acute and chronic PTSD in a significant minority. The underlying molecular mechanisms or outcome predictors determining these differences are not known. Molecular markers for identifying any mental disorder are currently lacking. Gene expression profiling during the triggering and development of PTSD may be informative of its onset and course. We used oligonucleotide microarrays to measure peripheral blood mononuclear cell (PBMC) gene expression of trauma survivors at the emergency room and 4 months later. Gene expression signatures at both time points distinguished survivors who met DSM-IV diagnostic criteria for PTSD at 1 and 4 months, from those who met no PTSD criterion. Expression signatures at both time points correlated with the severity of each of the three PTSD symptom clusters assessed 4 months following exposure among all survivors. Results demonstrate a general reduction in PBMCs' expression of transcription activators among psychologically affected trauma survivors. Several differentiating genes were previously described as having a role in stress response. These findings provide initial evidence that peripheral gene expression signatures following trauma identify an evolving neuropsychiatric disorder and are informative of its key clinical features and outcome. Replications in larger samples, as well as studies focusing on specific markers within the signatures discovered, are warranted to confirm and extend the diagnostic utility and pathogenetic implications of our results.