Epidemiological research serves a critical role in public mental health planning in the aftermath of disasters, particularly via estimation of the mental health burden and potential needs of affected communities. However, different measures are used across studies to assess mental health response, making cross-study comparison difficult. The National Women's Study Posttraumatic Stress Disorder module (NWS-PTSD) and PTSD Checklist (PCL) have been among the most widely used measures of PTSD in postdisaster research. Here, the authors used a sample of 233 New York City-area residents who were administered both the NWS-PTSD and PCL 4 months after the September 11, 2001 terrorist attacks. The PCL yielded higher prevalence estimates at the symptom, cluster, and diagnostic levels. Implications for the interpretation of epidemiological data are discussed.