Epidemiologically, disasters represent multiple forms of possible exposures, including exposure type (eg, natural versus human-made), intensity, and duration. It has been suggested that the consequences of human-made disasters (eg, terrorist incidents) may be more severe than those of natural disasters; recent evidence suggests that there may be a high prevalence of posttraumatic stress disorder (PTSD) among both direct survivors of such attacks and in the general population. Several studies after the September 11, 2001, terrorist attacks found that the prevalence of PTSD was higher in New York City than it was in the rest of the US and that there was a substantial burden of PTSD among persons who were not directly affected by the attacks. This raises important questions about the meaning of "exposure" to a disaster. Using data from an assessment of PTSD in the first 6 months after September 11th we considered the nature of the PTSD experienced by persons who were not directly affected by the September 11th attacks. These data suggest that persons in the general population may have clinically important posttraumatic stress symptomatology after a mass terrorist incident. Future research should consider mechanisms through which persons in the general population may be at risk for PTSD after such incidents.