We assessed the health of workers exposed to the World Trade Center (WTC) site and of a comparison group of unexposed workers, by means of a mail survey. Exposed workers reported higher frequency of symptoms consistent with posttraumatic stress disorder (PTSD), depression, anxiety, and other psychological problems, approximately 20 months after the disaster. PTSD was positively associated with traumatic on-site experiences and with respiratory problems. These findings may have important clinical and public health implications.