Many studies attempt to gauge population prevalence of post traumatic stress disorder (PTSD) following a major catastrophe. However, little is known about the validity of these estimates in the increasingly common situation of ongoing trauma. During the period of the Second Intifada in Israel, which involved frequent and widespread terrorist attacks, several studies generated estimates of PTSD prevalence in the Israeli population, but yielded widely discrepant findings. A number of focused surveys using symptom checklists estimated population prevalence of PTSD or probable PTSD diagnosis at about 9%. However a large population health study conducted during this same time period using a structured diagnostic interview yielded a very low estimated prevalence of PTSD (0.5%). We examine methodological differences that might account for these striking discrepancies. Inherent limitations of both checklist and structured interview methods may be especially problematic in the context of ongoing trauma. Findings regarding PTSD and associated factors obtained during ongoing trauma should be interpreted with caution.