The world's oldest profession may also be among its most traumatizing. A new study has found that a serious psychiatric illness resulting from exposure to physical danger is more common among prostitutes than among troops who have weathered combat duty.

The illness, post-traumatic stress disorder, is the modern equivalent of shell shock, or combat fatigue. It leaves survivors of serious physical danger emotionally numb, and tortured by recurrent nightmares and flashbacks, often for decades.

In a study to be presented today at the annual meeting of the American Psychological Association in San Francisco, researchers interviewed almost 500 prostitutes from around the world and discovered that two-thirds suffered from post-traumatic stress disorder. In contrast, the condition is found in less than 5 percent of the general population. Studies of veterans of combat in the Vietnam War have found that the disorder may be diagnosed in 20 percent to 30 percent, about half of whom have long-term psychiatric problems.

"Essentially, we need to view prostitution itself as a traumatic stressor," said Dr. Melissa Farley, a psychologist and researcher at the Kaiser-Permanente Medical Center in San Francisco who directed the study with colleagues from Turkey and Africa.

Dr. Farley's team interviewed male and female prostitutes ranging in age from 12 to 61 who operated on the street and in brothels in San Francisco and six large cities in Europe, Asia and Africa. The vast majority reported having sustained recurrent physical or sexual assaults in working hours.

Using a severity scale developed by scientists who study post-traumatic stress in the military, Dr. Farley's team found that the prostitutes averaged a slightly more severe form of the disease than even Vietnam veterans seeking treatment for the condition. That is an "enormously high" rating on the scale, said Dr. Matthew Friedman, executive director of the Department of Veterans' Affairs National Center for post-traumatic stress disorder and a professor of psychiatry at Dartmouth Medical School.

The frequency of post-traumatic stress disorder among the prostitutes appeared to be unrelated to their nationality or where they worked. It was as common in Istanbul as in San Francisco, and as common among the men and women working in three expensive brothels in Johannesburg as among those working in the streets of that city, even though less physical violence occurred in the brothels, Dr. Farley said.

Hers is the first study of post-traumatic stress in prostitutes, but other studies have shown similarly high frequencies of the disorder in other disadvantaged groups of women, including pregnant drug users, and homeless and battered women.

Dr. Farley also found that about two-thirds of the prostitutes studied complained of medical problems. Unexpectedly, few of the problems appeared to be related to sexually transmitted diseases, she said. Instead, they were similar to the medical problems researchers have found in other groups of patients with the disorder.

In contrast to the romantic vision of prostitution often presented by Hollywood, "prostitution is not just a job choice," Dr. Farley said. More than 90 percent of the prostitutes in her study said they "wanted out" of that way of life.