This study examined the long-term mental health outcomes of 2,960 nonrescue disaster workers deployed to the World Trade Center site in New York City following the September 11, 2001 (9/11) terrorist attacks. Semistructured interviews and standardized self-report measures were used to assess the prevalence of posttraumatic stress disorder (PTSD) and other psychopathology 4 and 6 years after the attacks. Clinician-measured rates of PTSD and partial PTSD 4-years posttrauma were 8.4% and 8.9%, respectively, in a subsample of 727 individuals. Rates decreased to 5.8% and 7.7% for full and partial PTSD 6 years posttrauma. For the larger sample, self-report scores revealed probable PTSD and partial PTSD prevalence to be 4.8% and 3.6% at 4 years, and 2.4% and 1.8% at 6 years. Approximately 70% of workers never met criteria for PTSD. Although PTSD rates decreased significantly over time, many workers remained symptomatic, with others showing delayed-onset PTSD. The strongest predictors of ongoing PTSD 6 years following 9/11 were trauma history (odds ratio (OR) = 2.27, 95% confidence interval (CI) [1.06, 4.85]); the presence of major depressive disorder 1–2 years following the trauma (OR = 2.80, 95% CI [1.17, 6.71]); and extent of occupational exposure (OR = 1.31, 95% CI [1.13, 1.51]). The implications of the findings for both screening and treatment of disaster workers are discussed.