New York:

Every so often, seemingly normal people suddenly walk out of their lives and disappear, with no recollection of who they are, where they are from or what their previous life was like. It is the stuff of fiction, but it happens in real life too.

Last year a Westchester County lawyer -a 57-year-old husband and father of two, Boy Scout leader and churchgoer — left the garage near his office and disappeared. Six months later he was found living under a new name in a homeless shelter in Chicago, not knowing who he was or where he came from.

Library searches and contact with the Chicago police did not help the man. His true identity was uncovered through an anonymous tip to “America’s Most Wanted.” But when he was contacted by his family, he had no idea who they were.

On the fictional side is a play called “Fugue,” now on stage at the Cherry Lane Theater in New York. In it, a psychiatrist interviews a woman found wandering homeless in Chicago. She does not know her name and can recall nothing about her life before landing in Chicago. The rest of this most interesting play by Leonora Thuna is an exploration of a rare but intriguing emotional disorder, known technically as dissociative fugue or dissociative amnesia.

A Sudden Change

People with this problem suddenly and unexpectedly take leave of their usual physical surroundings and embark on a journey that can last as little as a few hours or as long as several months. During the fugue state, individuals completely lose their identity, later assuming a new one. They don’t know their real names or anything about their former lives, and they do not recognize friends or family. They may not even remember how they got to where they are.

While loss of memory can occur for many reasons, dissociative fugue has no direct physical or medical cause. Rather, it is precipitated by a severe stress or emotionally traumatic event that is so painful the mind seems to shut down and erase everything, like a failed computer hard drive.

But unlike a computer whose unsaved information is lost forever, most if not all patients suffering from dissociative fugue eventually recover their memories, typically just as suddenly as the memories disappeared. While in the fugue state, people are unaware that their identity and memory have been lost, said David Schacter, professor of psychology at Harvard. They wander off, often traveling far from home.

It is only when they are forced to reveal some piece of biographical information that they realize they do not know who they are, which may lead to a desperate search to uncover their identity.

In a telling case detailed by Berton Roueché first in The New Yorker and later his book “The Medical Detectives, Volume II” more than a half-century ago, a man who felt increasingly trapped in his father-in-law’s business one day failed to show up at the store in Boston and later found himself in New York. Not until he had to provide his name for a hotel did he discover he did not know who he was.

After many failed attempts to uncover his identity, his past revealed itself while he was being quizzed by a doctor at Bellevue Hospital, he recalled. “All of a sudden, I knew, I remembered. I jumped up and shouted. I yelled, ‘I know — I can remember! I remember my wife’s name. It’s Mildred. We live in Boston. I can even tell you the address. And my name is Uhlan. Walter Uhlan.’

A Diagnostic Challenge

Elkhonon Goldberg, clinical professor of neurology at New York University and the author of three books on the human mind, says that some underlying neurological problem is the usual cause of amnesia. When amnesia has a physical basis, memory loss is usually not complete, but rather covers a part of someone’s life. The more recent memories are often lost, while memories of more distant events are preserved.

When examining a patient with memory loss that has no obvious physical cause, the first step, Dr. Goldberg said, is to look for a neurological cause like a head injury, stroke, viral encephalitis or temporal lobe epilepsy. In such cases, in addition to incomplete memory loss, there is usually a loss of individual facts like biographical information. However, when memory loss includes generic knowledge about whole classes of things, like how many wings birds have, the underlying cause is more likely to be psychogenic, Dr. Goldberg said.

When amnesia has an organic cause, people’s memories of who they are usually are not disrupted, Dr. Schacter said. Nor are memories usually lost of events that occurred before the physical problem. But such people may be unable to form new memories.

And sometimes cases of fugue have a precipitating psychological cause along with some form of underlying brain damage that is revealed, say, through a functional M.R.I. or PET scan, according to Morris Moscovitch, a psychologist at the University of Toronto.