American Indians are at high risk for exposure to violence and other traumatic events, yet few studies have investigated posttraumatic stress disorder (PTSD) or its neurobiological consequences in Indian communities. In the present study, a sample of American Indians (n = 146) were given a structured diagnostic interview that additionally indexed traumatic life events and symptoms emerging following those events. Electroencephalogram (EEG) spectra and visual event-related potentials (ERPs) to happy, sad, and neutral faces were also recorded from each participant. Ninety-nine percent of the sample had experienced at least one category of trauma with the mean number being 5, 27% had experienced at least 8 categories, and 13% met DSM-IV criteria for PTSD. The PTSD group did not differ on any demographic or diagnostic variables from the larger sample. An electrophysiological signature for PTSD was found that included increases in high-frequency gamma activity (20-40 Hz, F = 8.7, P < 0.004) in frontal leads, higher N1 amplitudes to sad stimuli in frontotemporal leads (F = 12.4, P < 0.001, F = 5.0, P < 0.03), and longer latency P3 components to happy stimuli in midline, central, and right frontal leads (F = 4.7, P < 0.03; F = 4.1, P < 0.04; F = 4.0, P < 0.05). These findings were observed in participants with PTSD, but not in a group with equivalently high trauma counts. These findings suggest that PTSD is associated with EEG hyperarousal, higher attentional levels to sad stimuli, and slower processing of happy stimuli. They also partially confirm ERP data reported in combat victims with PTSD suggesting that PTSD may induce neurobiological consequences that transcend type of eliciting trauma as well as ethnic and cultural factors.