The researchers did not find the results surprising, because the military has a new mental health screening program for returning soldiers and is encouraging them to get help early to prevent serious problems later, said study co-author Dr. Charles Hoge, a colonel at the Walter Reed Army Institute of Research.
Because of the new screening program, the findings cannot be compared with those from previous wars, Hoge said.
"There are psychological consequences of war and we want to address those up front," Hoge said. "The hope is we won't have as high rates of mental health consequences as we've seen in prior wars."
But CBS News correspondent David Martin reports the actual number of soldiers who need treatment is higher, since other studies have shown that half the soldiers who need treatment are unwilling to admit it.
"There are individuals who are afraid to come in and get help despite needing it, because of fear that they'll be stigmatized," Hoge told Martin.
Nineteen percent of those back from Iraq reported mental health concerns, compared with 11 percent of those back from Afghanistan and 8.5 percent of those returning from other places, such as Bosnia.
The study appears Wednesday in the Journal of the American Medical Association.
Veterans' advocates said the study supports their call for increased spending on mental health care for Iraq veterans.
President Bush's budget plan includes a 6 percent increase in spending for the Department of Veterans Affairs. But some in Congress say that is not enough because the increase hinges on more than $1 billion in cuts in other VA spending and the approval of new fees and co-payments for some veterans.
"This budget would ultimately shortchange veterans who need mental health services," said Ralph Ibson, a vice president of the National Mental Health Association. "This study can and should be a wake-up call in terms of veterans' mental health needs."
Medical authorities first accepted post-traumatic stress disorder as a psychiatric condition in 1980 at the urging of Vietnam veterans. A previous study by Hoge and his colleagues found 15 percent to 17 percent of soldiers returning from Iraq showed signs of the disorder, and many were reluctant to seek help because of the stigma attached to mental illness.
Shortly after starting the ground war in Iraq in 2003, the Pentagon began requiring returning service members to complete a three-page survey that is used to decide who needs additional help.
Among other things, the veterans are asked whether they have had nightmares, whether they are constantly on guard or easily startled, and whether they feel numb or detached from others.
"In prior wars, mental health issues weren't studied until years, sometimes decades, after the soldiers came back," Hoge said. "For this war, we're doing it differently. Research is influencing policy and we're adjusting policies as the data come in."