He was a 27-year-old former Marine, struggling to adjust to civilian life after two tours in Iraq. Once an A student, he now found himself unable to remember conversations, dates and routine bits of daily life. He became irritable, snapped at his children and withdrew from his family. He and his wife began divorce proceedings.

This young man took to alcohol, and a drunken car crash cost him his driver’s license. The Department of Veterans Affairs diagnosed him with post-traumatic stress disorder, or P.T.S.D. When his parents hadn’t heard from him in two days, they asked the police to check on him. The officers found his body; he had hanged himself with a belt.

That story is devastatingly common, but the autopsy of this young man’s brain may have been historic. It revealed something startling that may shed light on the epidemic of suicides and other troubles experienced by veterans of wars in Iraq and Afghanistan.

His brain had been physically changed by a disease called chronic traumatic encephalopathy, or C.T.E.

The discovery of C.T.E. in veterans could be stunningly important. Sadly, it could also suggest that the worst is yet to come, for C.T.E. typically develops in midlife, decades after exposure. If we are seeing C.T.E. now in war veterans, we may see much more in the coming years.

Dr. Stern emphasized that the study of C.T.E. is still in its infancy. But he said that his hunch is that C.T.E. accounts for a share — he has no idea how large — of veteran suicides. C.T.E. leads to a degenerative loss of memory and thinking ability and, eventually, to dementia. There is also often a pattern of depression, impulsiveness and, all too often, suicide. There is now no treatment, or even a way of diagnosing C.T.E. other than examining the brain after death.

So far, just this one case of a veteran with C.T.E. has been published in a peer-reviewed medical journal. But at least three groups of scientists are now conducting brain autopsies on veterans, and they have found C.T.E. again and again, experts tell me. Publication of this research is in the works.