Hypnotized people commonly recall falsehoods as facts, and this makes the widespread and increasing use of hypnosis by police departments an "extremely risky business," according to a new study by two Canadian researchers.

An estimated 5,000 to 10,000 police officials have been trained to hypnotize witnesses to improve their recall, despite doubts among psychologists and psychiatrists that hypnosis can be used to gain true information, uncontaminated by the process of hypnosis.

The new study is the first controlled effort to test the truth of facts recalled by hypnotized subjects, and to press subjects during questioning in the same manner as a police officer would.

In the study, 54 persons were shown a series of pictures and allowed a few seconds to concentrate on each. They then tried to recall them in daily tests. After a week, half the group was hypnotized in an effort to improve their recall, according to study authors Jane Dywan of St. Joseph's Hospital in Hamilton, Ontario, and Kenneth Bowers of Waterloo University in Waterloo, Ontario. The results are to be reported in the Oct. 14 editions of Science magazine.

Both groups at first correctly recalled about 30 of the 60 black-and-white pictures. After hypnosis, subjects recalled six or more items that they hadn't remembered earlier. But about five of the six items were false, even though the subjects had vivid memories of them.

In a second study, Dywan reported, about 90 percent of new "facts" recalled under hypnosis were false.

Those who were not hypnotized also recalled additional items merely with encouragement from the researchers--an average of 2 1/2 items apiece. Typically, two of the newly recalled items were false.

Dywan said that one of the most worrisome problems with hypnosis is that people who previously said they were unsure of a fact or a memory become very confident of it after they are hypnotized.

Researchers believe this confident attitude is one of the most important factors that juries and judges use to weigh the veracity of witnesses. Elizabeth Loftus, a psychologist at the University of Washington and an expert on the psychology of eyewitness testimony, said that hypnotized individuals may become contaminated witnesses because they may become sure of even their most shaky memories.

Most states have no legal precedents on the subject, and those that exist are conflicting. But state courts have increasingly begun to limit testimony from hypnotized witnesses or to require guidelines to assure that witnesses' memories are not contaminated during hypnosis, according to Arizona Assistant Attorney General Michael D. Jones, who recently fought a hypnotized-witness case up to the U.S. Supreme Court, which refused to rule on it.

Martin T. Orne, director of the unit for experimental psychiatry at the Institute of Pennsylvania Hospital and an expert in the use of hypnosis, explained the problem:

"The police are trying to help witnesses and victims to remember. So suppose we have a bank robbery. The witness did not really see the robber go away from the bank, but he knows the police think the robber was a short, bearded man. The witness comes to believe this, but when put on the stand, and the attorney asks, 'Did you actually see the man run away?' he must answer no.

"But now you take the same fellow and you hypnotize him. In hypnosis, he recalls the scene to the point where he can actually believe he sees a short, bearded man running away from the bank.

"What happens is that we may translate beliefs into memories. And since we weigh eyewitness testimony so heavily, that can create havoc in our system," Orne said.

Working with a witness who has few specific memories, it is possible for a hypnotist to unwittingly suggest memories and "create a witness" who has a number of crucial and vivid memories, he said.

From hypnotized witnesses can be gleaned "some new information, some of it accurate, some inaccurate," Orne said. "But neither the subject nor the hypnotist can tell which is which. Laymen and indeed law enforcement officials tend to believe it is much more accurate than it is, because the witness actually seems to relive it or see it before his very eyes. That is the catastrophe."

"It is such a compelling experience to see witnesses recall these things, even veteran investigators can be fooled," he said.

On the other side, Arizona Assistant Attorney General Jones said that, while it is important to take precautions in hypnotizing witnesses--such as taking a videotape of the hypnosis session and trying to assure that hypnotists do not accidentally plant suggestions in the witnesses' memories--hypnosis is "really not that much different than other ways of trying to get people to recall things."

He said police officers ordinarily try to get witnesses to relax, to have confidence in the questioner and to recall events by walking through them, techniques little different than those used in hypnosis.