Pentagon data shows that newly diagnosed cases of post-traumatic stress disorder hit the peak among US troops who served in Iraq and Afghanistan in 2007.
Statistics show a nearly 50 percent jump in new cases last year when 13,951 service members were diagnosed with combat stress, compared to 9,549 in 2006.
The increase was probably the result of "the high operations tempo, if you look at what was going on last -- the fierce fighting and high casualties," said Lieutenant Colonel Anne Edgecomb, an army spokeswoman.
The US surge strategy in Iraq peaked last year with more than 160,000 troops in country and major offensives against Al-Qaeda and other Sunni insurgents that made it the deadliest year of the war for US troops.
Since 2003, more than 39,000 US troops have been diagnosed with post traumatic stress disorder, according to the military statistics, which were produced by the Defense Manpower Data Center and the Military Medical Data Repository.
Symptoms include hyper-vigilance, intrusive thoughts, flashbacks, numbness, and nightmares lasting more than 30 days that disrupt a person's ability to work or function socially.
Lieutenant General Eric Schoomaker, the army's surgeon general, said studies show that 15 to 30 percent of deployed soldiers are susceptible to experiencing combat stress.
"We expect that the majority of those folks, if found early enough and if treated appropriately, will have resolution of their symptoms and not go on to have a chronic disorder," he told reporters on Tuesday.
But Schoomaker said the full dimension of the problem is difficult to determine because some returning soldiers do not have access to health care, and symptoms arising three to six months after deployment can go unrecognized.
"I think we are in our infancy right now of really fully knowing what the extent of this is, and how best to track," he said.