An influx of 18-to-34-year-olds, many of them confused relics of the drug culture of the 1960s and '70s, has started refilling the nation's mental hospitals.

Their huge and surprising number, accounting for approximately 162,000 hospital admissions each year, threatens to reverse more than a quarter-century of steady decline in state mental hospital populations.

An estimated 42 percent of the new patients are in the 18-to-34 age group, according to a nationwide survey by the National Institute of Mental Health. Nearly three in five are young men, some "so regressed they can no longer relate to other human beings," one state hospital official said.

"What we seem to be seeing is a major turning point" in what had been a depopulation of the state hospitals, Dr. Carl Taube of the NIMH said.

"Two things are happening," said Dr. Ronald Manderscheid of the NIMH. "There are for the first time in many years more additions than separations from the hospitals, not counting deaths. It is the deaths of the older, longtime patients that are still lowering state hospital censuses. And the elderly who are dying off are being replaced by this group that people are calling the 'new' or 'young chronics.' "

There probably are many reasons for this "large growing army" of younger mental patients, said Abbott Weinstein, director of analytical services in the New York Office of Mental Health.

"These are the children of the baby boom population," the 64 million children born between 1946 and 1961, he said. Now they have entered one of the highest risk age groups for mental disorders, especially schizophrenia.

"This is the population that went through the '60s and '70s with all kinds of dislocations in their lives, though how many would have become mentally ill anyway no one can say," said Weinstein, who added that the new patients are "sicker, more hostile and more difficult to treat."

"They don't act like the old passive, compliant long-term patients," said Dr. Leona Bachrach, a University of Maryland sociologist. "Many are vocal, demanding, aggressive. The majority are very noncompliant and resistant to treatment."

According to those treating them, many of these patients continue to abuse drugs or drink anything alcoholic, or refuse drug treatment or other standard therapies.

Their illnesses, however, are generally the same as those of other mental patients: schizophrenia, the loss of touch with reality that one psychiatrist has called "shattered personality," severe depression or manic-depressive illness, personality or behavioral disorders like paranoia and extreme compulsions.

The trend has only recently begunto be discussed in mental health journals and reports at psychiatric meetings.

Bachrach, who reported on "young chronics" in the journal Hospital and Community Psychiatry, said that although many of these patients are counterculture graduates, "I don't think they're getting schizophrenia for that reason. In fact, I think some of them were just hidden in the counterculture, and now there's no one to protect them any more and they've become very visible.

"They're by no means all in the hospital," she added. "They are actually extremely mobile, and move from city to city, living any way they can and putting tremendous demands on local services for food, shelter, clothing and medical care. They're not so much the old '60s counterculture as their own culture."

Accommodating these new patients in hospitals presents problems, according to Dr. Thomas Krajewski, superintendent of Maryland's Springfield State Hospital at Sykesville, who said, "They seem to require more than we have available.

"We are certainly trying the usual methods: individual and family and group therapy, medication, social and activation therapies and rehabilitation," Krajewski said. "But many are resistant. Many have multiple problems, including drug use. Or brain damage due to drug use.

"Some have lost very basic skills. Some don't even know how to dress. Some are so regressed, so isolated, that we've begun giving them pets so they can begin to take responsibility for living things, with the hope this can be escalated to taking responsibility for themselves. We're trying different approaches to see what will work."

In 1955, state and county mental hospitals housed 559,000 patients, a record number. New drugs--and a new philosophy that called for home-town hospital or short-term clinic care--started emptying the hospitals until, in 1980, there were just 132,164 resident patients.

It had appeared that this trend would continue, until the new wave of admissions began.

"It is certain," Krajewski said, "that many of these patients will be here an extended period of time."