The purpose of this study was to examine electrophysiological response to trauma-relevant stimuli in combat-related post-traumatic stress disorder (PTSD). Study design incorporated comparison of 10 Vietnam War veterans with PTSD diagnosis to 10 Vietnam War veterans with no mental disorder diagnosis on P3 components in a series of two oddball tasks (trauma-relevant threat, trauma-irrelevant threat) counterbalanced for order. Each task included high probability emotionally neutral distractor words and low probability neutral target words, but differed in the content of low probability threat words. Whereas threat words in the trauma-relevant oddball task pertained directly to combat trauma, threat words in the trauma-irrelevant oddball task were socially threatening words. Results revealed that, in comparison to healthy combat veterans, those diagnosed with PTSD demonstrated: (a) attenuated P3 response to neutral target items at selected electrode sites across both oddball tasks; and (b) increased responsivity to trauma-relevant combat stimuli but not to trauma-irrelevant social-threat stimuli at frontal electrode sites (F3, F4). Results are consistent with resource allocation models of PTSD, which suggest that PTSD is characterized by attentional bias to threat stimuli at the expense of attention to emotionally neutral information.