Background: A central problem in posttraumatic stress disorder (PTSD) is a reduced capacity to suppress fear under safe conditions. Previously, we have shown that combat‐related PTSD patients have impaired inhibition of fear‐potentiated startle (FPS). Given the high comorbidity between PTSD and depression, our goal was to see whether this impairment is specific to PTSD, or a non‐specific symptom associated with both disorders. Methods: Fear‐potentiated startle was assessed in 106 trauma‐exposed individuals divided into four groups: (a) No diagnosis control, (b) PTSD only, (c) major depression (MDD) only, and (d) comorbid PTSD and MDD. We used a novel conditional discrimination procedure, in which one set of shapes (the danger signal) was paired with aversive airblasts to the throat, and different shapes (the safety signal) were presented without airblasts. The paradigm also included fear inhibition transfer test. Results: Subjects with comorbid MDD and PTSD had higher FPS to the safety signal and to the transfer test compared to controls and MDD only subjects. In contrast to the control and MDD groups, the PTSD and comorbid PTSD and MDD groups did not show fear inhibition to safety cues. Conclusions: These results suggest that impaired fear inhibition may be a specific biomarker of PTSD symptoms. Depression and Anxiety, 2010. © 2010 Wiley‐Liss, Inc.