The study, published in this week's New England Journal of Medicine, is one of a very few that have examined the psychological impact of war so close to the time of deployment. It has already begun to reshape how soldiers are treated, both in the field and after they return home, researchers said.

Specifically, the study was partly aimed at gauging the psychological barriers -- from concern about the stigma associated with mental illness to fear of possible harm to one's career -- that keep troops from seeking help for combat-related stress disorders.

"There are a significant number of soldiers who need or require help," study co-author Carl Castro of the Walter Reed Army Institute of Research said in an interview.

Castro and his colleagues studied members of four US combat infantry units using an anonymous survey administered either before deployment to Iraq or three to four months after their return from combat duty. Troops who served in Afghanistan were also included in the survey.

The rate of post-traumatic stress disorder was 5 percent among the 2,530 Army infantry soldiers headed to Iraq, but it rose sharply following deployment, the researchers found.

The rate of the disorder was nearly 13 percent among 894 soldiers who returned from eight months of duty in Iraq. It was more than 12 percent for 815 Marines who served six months there, and about half that for 1,962 infantry soldiers who returned from Afghanistan. The surveys were done in 2003.

In contrast, the rate of long-term post-traumatic stress disorder among Vietnam War veterans years after their return has been 15 percent, and between 2 percent to 10 percent among veterans of the Gulf War.