A number of factors, including subjective reactions and appraisal of danger, influence one's reaction to a traumatic event. This study used telephone survey methodology to examine adolescent and parent reactions to the 2001 World Trade Center attacks 6 to 9 months after they occurred. The prevalence of probable posttraumatic stress disorder (PTSD) in adolescents was 12.6%; 26.2% met study criteria for probable subthreshold PTSD. A probable peri-event panic attack in adolescents was strongly associated with subsequent probable PTSD and probable subthreshold PTSD. This study suggests that the early identification of peri-event panic attacks following mass traumatic events may provide an important gateway to intervention in the subsequent development of PTSD. Future studies should use longitudinal designs to examine the course and pathogenic pathways for the development of panic, PTSD, and other anxiety disorders after exposure to disasters.