Most veterans who had persistent post-traumatic stress a decade or more after serving in the Vietnam War have shown surprisingly little improvement since then, and a large percentage have died, a new study finds, updating landmark research that began a generation ago. Members of minorities who enlisted before finishing high school were especially likely to develop such war-related trauma, as were those veterans who had killed multiple times in combat, the study found.

The new analysis, financed by the Department of Veterans Affairs, is part of the first effort to track a large, nationally representative sample of service members through their adult lives, and it is likely to have implications for post-traumatic stress treatment and disability-benefit programs for years to come, the authors said. Both issues have been hotly debated during the drawdown from the wars in Iraq and Afghanistan.

The study, which was to be presented on Friday, confirms that a vast majority of veterans learn to cope. Yet most of those who do not — 11 percent, in the Vietnam sample — could live with traumatic stress for the remainder of their lives. An estimated 13 percent of current active-duty soldiers and 10 percent of Marines have post-traumatic stress disorder, or PTSD, characterized by disabling flashbacks, hyper-arousal and sleep problems, and about 120,000 sought treatment in 2012, according to government figures.

“This study shows us what the road ahead is going to look like,” said an author, Dr. Charles Marmar, chairman of psychiatry at NYU Langone Medical Center and director of the NYU Cohen Veterans Center. “A significant number of veterans are going to have PTSD for a lifetime unless we do something radically different.” More than 18 percent of those with PTSD had died by retirement age, about twice the percentage of those without the disorder.