Background Among police responders enrolled in the World Trade Center Health Registry (WTCHR), Post-traumatic Stress Disorder (PTSD) was almost twice as prevalent among women as men 2–3 years after the 9/11 attacks. Methods Police participants in the WTCHR Wave 1 survey 2–3 years after 9/11/01, were reassessed for probable PTSD at Wave 2, 5–6 years after 9/11/01, using PCL DSM-IV criteria. Results Police participants in the Wave 2 survey included 2,527 men, 413 women. The prevalence of “Probable PTSD” was 7.8% at Wave 1 and 16.5% at Wave 2. Mean PCL scores increased from 25.1 to 29.9 for men and 28.6 to 32.2 for women. Prevalence of PTSD was higher for women than for men at Wave 1 (χ2 = 10.882, P = 0.002), but not Wave 2 (χ2 = 2.416, P = 0.133). Other risk factors included losing one's job after 9/11 and being disabled. Conclusions Prevalence of probable PTSD among police doubled between 2003–2004 and 2006–2007. After the 2-year time span, the gender difference was no longer significant; prevalence of PTSD symptoms increased and there was a substantial amount of co-morbidity with other mental health problems. Further development of prevention and intervention strategies for police responders with symptoms of PTSD is needed. The observed upward trend in PCL scores over time in police officers with PCL scores less than 44, suggests that PTSD prevention and intervention strategies should be applied to all police affected by the 9/11 attacks, not limited just to those with PTSD symptoms.