Nearly a fifth of U.S. troops returning from the war in Iraq may suffer from post-traumatic stress disorder and other mental health problems, but many do not seek treatment, researchers report.

The study, published Thursday in the 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, the researchers said.

"There are a significant number of soldiers who need or require help," said co-author Carl A. Castro of the Walter Reed Army Institute of Research. He and his colleagues studied four U.S. combat infantry units using an anonymous survey administered either before deployment to Iraq or three to four months after their return. Troops in Afghanistan were also included.

The rate of post-traumatic stress disorder was 5 percent among the 2,530 Army infantry soldiers headed to Iraq but rose to 13 percent among 894 soldiers who returned from eight months of duty in Iraq.

The rate was more than 12 percent for 815 Marines who served six months there, and about half that for 1,962 infantry troops who returned from Afghanistan.