Environmental stressors such as mass disasters may contribute to an increased prevalence of depression within the population affected. We examined the prevalence of probable major depression and risk factors for depression in the 6-month period after the September 11, 2001, attacks on the World Trade Center among New York City (NYC) metropolitan residents. A total of 2700 persons who were representative of the NYC metropolitan area were included in this cross-sectional telephone survey. The prevalence of probable major depression in the 6 months after the attacks was 9.4%. Multivariate logistic regression covariates associated with the likelihood of probable major depression included being directly affected by the attacks, having a perievent panic attack, experiencing multiple life stressors, and having been exposed to previous traumatic events. Mass traumatic event exposure appears to be an independent environmental risk factor for depression in the postdisaster context; specific reactions such as perievent panic attacks may have prognostic value.