The impact of the September 11th terrorist attacks on adolescents' mental health is reported. Two successive cohorts of students in 6 New York state high schools, identified from health courses, completed an in-school self-report baseline assessment of hopelessness, impairment, and help-seeking behavior. One year later, these students completed a follow-up telephone interview consisting of a comprehensive psychiatric assessment and a readministration of the baseline survey. The first cohort completed the baseline in 1999 and the follow up in 2000; the second cohort completed the baseline in 2000 and the follow up in 2001. Because the September 11th attacks occurred approximately 1 month before the scheduled commencement of the second cohort's follow up, this "September 11th cohort" also completed questions about the teenagers' experiences of the September 11th terrorist attacks in the follow-up interview. Taking advantage of the data gathered in the first cohort prior to the September 11th attacks, this article presents results comparing the mental health of teenagers in the September 11th cohort (N = 362) to those in the first cohort (N = 429), serving as a control group. Although the majority of youths exhibited no untoward psychological consequences post attacks, a minority of youths, reporting initial responses of numbing after the attacks, exhibited clinically significant psychological morbidity. Youths sought more help from informal than formal sources in the attacks' immediate aftermath. The findings have direct relevance for developing optimal practices to assist vulnerable youths in the aftermath of a terrorist attack. (Contains 5 tables.)