This study attempted to replicate findings of abnormal auditory event-related potentials (ERPs) in posttraumatic stress disorder (PTSD) in a sample of Vietnam combat veterans. Veterans with combat-related PTSD, divided into unmedicated (unmed-PTSD, n = 12) and medicated (med-PTSD, n = 22) groups, and veterans without PTSD (non-PTSD, n = 10) completed a three-tone "oddball" target detection task while ERPs were measured. Individuals with comorbid panic disorder (PD) were excluded from the primary analyses. Parietal P3 amplitude to the target tone was significantly smaller in unmed-PTSD compared to med-PTSD and non-PTSD groups. These differences did not remain significant when an adjustment was made for level of depression. Parietal P3 amplitude was also negatively correlated with state anxiety. Secondary analyses within the unmed-PTSD group indicated that participants with comorbid PD (n = 3) had the largest parietal P3 amplitudes to target tones. Results are consistent with attentional or concentration deficits in PTSD and highlight the importance of considering comorbid diagnoses. The absence of ERP differences between med-PTSD and non-PTSD participants suggests that psychotropic medication may normalize these deficits.