The survey also showed that less than half of those with problems sought help, mostly out of fear of being stigmatized or hurting their careers.

The survey of Army and Marine combat units was conducted a few months after their return from Iraq or Afghanistan last year. Most studies of past wars' effects on mental health were done years later, making it difficult to compare the latest results with those from the Vietnam or Persian Gulf wars, said Dr. Charles W. Hoge, one of the researchers at the Walter Reed Army Institute of Research.

Of particular concern, he said, is that troops with problems are not seeking care.

"The most important thing we can do for service members who have been in combat is to help them understand that the earlier that they get help when they need it, the better off they'll be," Hoge said.

The study is published in Thursday's New England Journal of Medicine.

Once called shell shock or combat fatigue, post-traumatic stress disorder can develop after witnessing or experiencing a traumatic event. Symptoms include flashbacks, nightmares, feelings of detachment, irritability, trouble concentrating and sleeplessness.

Dr. Matthew J. Friedman, executive director of the Department of Veterans Affairs' National Center for Post-Traumatic Stress Disorder, said it is remarkable to have the study's results available while there are still troops in Iraq and Afghanistan. But he said he believes the estimates are conservative and it may be too early to determine the extent of mental problems.

"I'm not an alarmist, but I think this is a serious problem. It may be worse just because of the nature of the war," he said, citing extended tours of duty and the change of mission from liberation to occupation.

In the study of 6,201 service members, the researchers surveyed four different groups: Army brigades before they went to Iraq, after six months in Afghanistan and after eight months in Iraq; and Marine battalions after six months in Iraq.

The soldiers and Marines filled out anonymous questionnaires asking about their mental health, their use of mental health services and their combat experience. The returning troops took the survey three to four months after coming home.

Only active-duty combat troops were questioned.

Symptoms of major depression, anxiety or post-traumatic stress disorder were reported by 16 percent to 17 percent of those who served in Iraq, 11 percent of those who were in Afghanistan and 9 percent questioned before they left.

The differences were greatest for post-traumatic stress disorder with about twice as many with PTSD after Iraq (12 percent) than Afghanistan (6 percent). Before deployment, the rate was 5 percent, about the same as the general U.S. population.

The troops in Iraq saw more combat, including firefights and attacks, than those in Afghanistan. The Iraq units took part in the early fighting of the war.

Studies done years after the Vietnam and Persian Gulf wars showed the rate of post-traumatic stress disorder at the time was 15 percent for Vietnam veterans and 2 percent to 10 percent for Gulf War veterans, the researchers reported.

In the latest study, only 38 percent to 40 percent of those who indicated mental health disorders were interested in getting help, and 23 to 40 percent reported seeing someone for help. They cited concerns about how they would be seen by peers and potential damage to their careers.

The study points up the need to "reduce the barriers and make it more likely for people to come in and get the help that they need," Hoge said.

In an editorial in the journal, Friedman said members of the military are skeptical that a decision to seek counseling can remain confidential. The study's participants "apparently were afraid to seek assistance for fear that a scarlet P would doom their careers."

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