Many of those affect have not recovered, The New York Times reports, citing figures compiled by three Sept. 11, 2001, health programs.
Those with the disorder suffer a wide range of symptoms -- inability to sleep, replaying the attacks in their minds, nightmares, difficulty concentrating, jitters and over-reaction to alarms or loud noises.
Some avoid anything that reminds them of the day of the attacks on the twin towers, the Times said.
Doctors predict an increase in PTSD symptoms as the 10th anniversary of the attacks nears.
Millions of dollars for treating victims of the disorder will be spent in the next few years under a measure passed by Congress in December. But of the $4.3 billion the measure provides, people with PTSD can receive only treatment assistance, while those with Sept. 11-related physical problems can qualify for treatment and compensation, because of questions about the scope of PTSD diagnosis, which is just three decades old.
Charles Figley, professor of disaster mental health at Tulane University's School of Social Work -- and a former Marine who wrote of PTSD in a book on Vietnam War veterans -- said Sept. 11 trauma persists because it hit so close to home.
"It's the places you see every day, where you proposed to your wife, where you remember getting the news that you got promoted, where your young children played," Figley said. "You go into a combat zone and then you leave. You don't leave home. You return all the time."