Context: The September 11, 2001, attacks against the United States provide a unique opportunity to examine longitudinally the process of adjustment to a traumatic event on a national scale. Objective: To examine the degree to which demographic factors, mental and physical health history, lifetime exposure to stressful events, September 11-related experiences, and coping strategies used shortly after the attacks predict psychological outcomes over time. Design, Setting, and Participants: A national probability sample of 3496 adults received a Web-based survey; 2729 individuals (78% participation rate) completed it between 9 and 23 days (75% within 9 to 14 days) after the terrorist attacks. A random sample of 1069 panelists residing outside New York, NY, were drawn from the wave 1 sample (n=2729) and received a second survey; 933 (87% participation rate) completed it approximately 2 months following the attacks. A third survey (n = 787) was completed approximately 6 months after the attacks. Main Outcome Measures: September 11-related symptoms of acute stress, posttraumatic stress, and global distress. Results: Seventeen percent of the US population outside of New York City reported symptoms of September 11-related posttraumatic stress 2 months after the attacks; 5.8% did so at 6 months. High levels of posttraumatic stress symptoms were associated with female sex (odds ratio [OR], 1.64; 95% confidence interval [Cl], 1.17-2.31), marital separation (OR, 2.55; 95% Cl, 1.06-6.14), pre-September 11 physician-diagnosed depression or anxiety disorder (OR, 1.84; 95% Cl, 1.33-2.56) or physical illness (OR, 0.93; 95% Cl, 0.88-0.99), severity of exposure to the attacks (OR, 1.31; 95% Cl, 1.11-1.55), and early disengagement from coping efforts (eg, giving up: OR, 1.68; 95% Cl, 1.27-2.20; denial: OR, 1.33; 95% Cl, 1.07-1.64; and self-distraction: OR, 1.31; 95% Cl, 1.07-1.59). In addition to demographic and pre-September 11 health variables, global distress was associated with severity of loss due to the attacks (β=.07; P=.008) and early coping strategies (eg, increased with denial: β=.08; P=.005; and giving up: β=.05; P=.04; and decreased with active coping: β=-.08; P=.002). Conclusions: The psychological effects of a major national trauma are not limited to those who experience it directly, and the degree of response is not predicted simply by objective measures of exposure to or loss from the trauma. Instead, use of specific coping strategies shortly after an event is associated with symptoms over time. In particular, disengaging from coping efforts can signal the likelihood of psychological difficulties up to 6 months after a trauma.