BACKGROUND Police responders to the 2001 World Trade Center (WTC) disaster were previously reported to have an increased prevalence of probable posttraumatic stress disorder (PTSD). METHODS Four thousand seventeen police responders (3,435 men and 582 women) were interviewed 2-3 years after 9/11/01 as part of the World Trade Center Health Registry. Demographic, occupational, and event-specific risk factors were evaluated for probable PTSD, determined by DSM-IV criteria using the Posttraumatic Stress Checklist (PCL). RESULTS Overall prevalence of probable PTSD was 8.3% (women: 13.9%; men: 7.4%, P < 0.001). Risk factors for both genders included 9/11-related injury and older age. For men, specific risk factors were: presence in WTC Towers on 9/11 and Hispanic ethnicity; and for women, witnessing horror and education less than a college degree. CONCLUSIONS Significantly higher prevalence of probable PTSD was found for female police responders. Although consistent with civilian populations, this finding contrasts with other studies of PTSD and WTC rescue and recovery workers, and police prior to 9/11.