Although the short- and midterm psychological effects of the attacks on September 11, 2001 (9/11) have been well described, less is known about the long-term effects. This study examines the course of probable posttraumatic stress disorder (PTSD), its predictors and clinical consequences in a cohort of 455 primary care patients in New York City, interviewed approximately 1 and 4 years after 9/11. The rate of PTSD decreased from 9.6% to 4.1%. Pre-9/11 major depressive disorder emerged as the strongest predictor of PTSD, particularly late-PTSD. At follow-up, late-PTSD was associated with major depressive and anxiety disorders, and PTSD regardless of timing was associated with impaired functioning. Findings highlight the importance of ongoing evaluation of mental health needs in primary care settings in the aftermath of disasters.