Previous studies on sensory gating process in post-traumatic stress disorder (PTSD) have yielded conflicting results. To investigate sensory gating function in PTSD we performed a case-control study in a sample of 12 patients with PTSD related to urban violence, compared to 24 normal subjects and 12 schizophrenic subjects evaluating auditory mid-latency evoked potential P50 in a double-click paradigm as a measure of sensory gating. PTSD subjects showed poorer sensory gating as evidenced by higher P50 ratios as compared to normal subjects (85.6% vs. 44.4%, P=0.002). Test and conditioning amplitudes did not differ with statistical significance alone, suggesting a combined effect. Schizophrenic subjects had higher conditioning and marginally smaller test amplitudes when compared to healthy controls, but were not statistically different from PTSD subjects. The present study replicated previous findings of sensory gating dysfunction in PTSD. The pattern of this dysfunction resembles that found in schizophrenia, with both test and conditioning amplitudes possibly implicated. Further studies are still necessary to better understand the pathophysiology of this neurophysiological dysfunction and its nature as a trait or state marker. The P50 paradigm may also become an objective parameter to assess the effects of new treatments for PTSD.