New Yorkers who watched in horror as the World Trade Center towers collapsed on September 11 suffered symptoms of psychological distress greater than other Americans, according to a study published this week in the Journal of the American Medical Association.

Researchers were not surprised to find that New York City area residents reported the highest psychological stress levels of any Americans sampled in the immediate aftermath of the attacks. Eleven-percent of New Yorkers surveyed reported stress symptoms.

Three percent of residents surveyed in the Washington D.C. area, where a plane slammed into the Pentagon, similarly reported elevated stress.

Four-percent of Americans living in other parts of the country reported symptoms, as well.

The study by William Schlenger and colleagues of the Research Triangle Institute in Research Triangle Park, North Carolina attempted to measure the direct and indirect psychological impact of 9-11, the most deadly terrorist attacks in the history of the United States.

Mr. Schlenger says direct exposure to the terrorist acts made a difference in how New Yorkers and Washingtonians felt after the attacks.

"That is, the crashing of planes into the World Trade Center towers and the ultimate collapse of those towers was a much more extreme exposure than what happened in the Washington area," he explained.

The researchers surveyed nearly 2300 adults across the country in the two months after attacks on the World Trade Center and the Pentagon outside Washington, D.C. They were looking for symptoms of post traumatic stress disorder, a psychiatric illness in which sufferers re-live a traumatic event over and over again, may have trouble sleeping and often feel numb and detached.

The authors of the study conclude there could be more than 500,000 potential cases of post-traumatic stress disorder in the New York metropolitan area.

An editorial in JAMA cautions against calling stress reactions "symptoms" of post traumatic stress disorder. The editorial goes on to say that while the study provides a valuable look at the nation's mental health in the wake of 9-11, early stress reactions do not predict long term problems with post traumatic stress disorder for those who lived through the terrorist attacks.