In October 2001, the New York State Office of Mental Health and the Department of Epidemiology of the Mailman School of Public Health of Columbia University conducted a rapid assessment of the nature and magnitude of mental health needs in the state resulting from the September 11th terrorist attacks on the World Trade Center. This effort was carried out during a period of great turmoil and uncertainty as New Yorkers responded to the shocking events of this unprecedented disaster. Using the limited data available at the time, we estimated that over 520,000 persons in New York City and the surrounding counties would experience posttraumatic stress disorder resulting from exposure to the attacks, and that more than 129,000 would seek treatment for this disorder during 2002. This assessment is part of an ongoing collaborative process between public and academic partners; the effort is designed to strengthen the capacity of the mental health system to respond to current and future terrorism. Estimates from this initial assessment will be refined over time as further data concerning the impact of the September 11th attacks become available.