Four decades after the end of the Vietnam War, 11 percent of its U.S. veterans continue to have post-traumatic stress disorder (PTSD), according to new research updating a landmark study conducted in the 1980s.

At that time, researchers found that 15 percent of Vietnam veterans had PTSD.

The new findings, presented earlier this month at a meeting of the American Psychological Association, suggest that for a significant portion of Vietnam veterans the disorder is a chronic condition.

“They develop a very difficult-to-treat form of PTSD,” said Dr. Charles Marmar, a psychiatrist at New York University who worked on the original study and the follow-up, which was funded by the Department of Veterans Affairs.

Some experts not involved in the research, however, suggested the new estimate may be too high because it relied on a standardized questionnaire rather than a structured clinical interview that is considered the gold standard for diagnosing the disorder.

When the researchers used the interview method to assess a subset of veterans in the study, the PTSD rate fell to 4.5 percent.

“How one assesses PTSD affects one’s estimate of its prevalence,” said Richard McNally, a Harvard University psychologist and expert on the disorder.

The original research, known as the National Vietnam Veterans Readjustment Study, included 1,632 veterans who had been deployed to the war and 716 others who served during that era but never went to Vietnam.

Based on that sample, it is estimated that 31 percent of Vietnam veterans had suffered from PTSD at some point in their lives but that by the late 1980s about half no longer did.

Those findings were controversial because less than a third of service members in Vietnam were in combat roles or combat-support units. One re-analysis of the data years later found that 19 percent had PTSD at some point and that 9 percent were still suffering from it.

For the new study, the researchers tracked down the research subjects.

PTSD has long been associated with early death, so researchers were not surprised to find that among veterans who deployed to Vietnam, those who had the disorder in the 1980s were twice as likely as those without it to be dead today.

Roughly 1 in 4 had died. Their death rate from cancer was especially elevated, possibly because veterans with PTSD are more likely to smoke.

Of the 1,839 veterans from the original study who were still alive, 1,450 participated in the new research.

“The majority of people, even those in high combat, successfully adjust,” Marmar said.

But the study found that certain groups face an increased risk of developing PTSD: high-school dropouts, minorities, those who engaged in killing, and those who were very young when they served.