"I don't think right now we ... have good numbers," Army Surgeon General Eric Schoomaker said Tuesday.
That's partly because officials have been encouraging troops to get help even if it means they go to private civilian therapists and don't report it to the military. The 40,000 cases cover only those that the military has tracked.
Officials have estimated that roughly 50 percent of troops with mental health problems don't get treatment because they're embarrassed or fear it will hurt their careers.
An accounting of diagnosed cases released by Schoomaker to reporters Tuesday shows the hardest hit last year were Marines and Army soldiers, the two ground forces bearing the brunt of combat in Iraq and Afghanistan.
The Army reported more than 10,000 new cases last year, compared to more than 6,800 the previous year. More than 28,000 soldiers altogether were diagnosed with the disorder over the last five years.
The Marine Corps had more than 2,100 cases in 2007, compared to 1,366 in 2006. They have had more than 5,000 PTSD cases diagnosed since 2003.
Schoomaker attributed the big rise partly to the fact that officials started an electronic record system in 2004 that captures more information, and to the fact that as time goes on the people keeping records are more knowledgeable about the illness.
He also blamed increased exposure of troops to combat. Factors increasing combat exposure in 2007 included President Bush's troops buildup, increased violence in both wars and the fact that a number of troops are serving their second, third or fourth tours of duty -- a factor mental health experts says dramatically increases stress.
In order to supply enough forces for the buildup, officials also extended tour lengths to 15 months from 12, another factor that caused extra emotional strain.
Schoomaker said he believes PTSD is widely misunderstood by the press and the public -- and that what is often just normal post-traumatic anxiety and stress is mistaken for full-blown PTSD cases.
Experts say many troops have symptoms of stress that can be managed with treatment and should not be confused with cases that go untreated for a long time and those that develop into a mental disorder.
The Pentagon had previously only given a percentage of troops believed affected by depression, anxiety, stress and so on -- saying up to 20 percent return home with such symptoms. A recent private study estimated that could mean up to 300,000 of those who've served have symptoms.