NEW YORK -- Behind a locked, bright-yellow door on the 18th floor of Bellevue Hospital, a colorful impressionist landscape celebrates Christmas.

Trees and a house are drawn in vivid greens, blues and reds, with the greeting "Ho, Ho, Ho" painted in shaky brown letters almost running off the paper. On another wall is a drawing of a snowman with coal black buttons. It was done by the same artist, who reached back into her own life to name her creation.

"Homeless Snowman," the patient wrote.

It has been almost two months since New York City began removing from the streets and involuntarily hospitalizing gravely mentally ill homeless. Since Oct. 29, physicians and police officers have taken 22 men and 19 women, a few in handcuffs, to Bellevue's 18 West Adult Psychiatric Unit, with its spectacular views of Manhattan's skyline.

On 18 West, progress often is measured in inches.

Mayor Edward I. Koch, bearing treats, visited the ward on Thanksgiving. The first person he approached was a particularly vulnerable-looking man in his late 20s, who held both hands up defensively when the mayor came close.

"Would you take a cookie? Do you want a cookie?" Koch asked.

"I don't think I should," the man stammered.

The unit's executive director quietly explained that the patient was paranoid and probably thought the mayor was trying to poison him.

"Of course not, you shouldn't take the cookie now," Koch countered. "Just think about it. I'll be back."

During his tour of the unit, the mayor saw other patients playing checkers or watching television. He stopped to chat and all the others accepted his offer of food. Finally, on his way out, he passed the first patient again.

"Last chance," coaxed Koch. "Do you want the cookie?"

"I'll take it," the man replied, at last accepting and eating the food.

Although most progress is made in such painstaking small steps, for some patients there has been genuine improvement.

Four of the first 41 patients have been discharged, some into the care of long-lost relatives. Several have been shifted to other wards after physicians found medical rather than mental problems. Eight others have been transferred to a special 50-bed homeless unit set up by New York State at Creedmore Psychiatric Center for longer term care.

"One thing everyone learns is patience," said Dr. David Nardacci, the homeless unit's director. "We're dealing with people where changes are measured more slowly. . . . You have to be sort of content with what, to someone else, might seem to be a more minor victory, but nonetheless, a real victory.

"One patient who couldn't sit in a dining hall with other patients can now sit in the same room and eat lunch. Aside from just asking other patients for a cigarette, the patient can now ask someone else a personal question: 'Oh, why are you here?' "

Since Oct. 29, 18 West has become both a medical model and a public policy laboratory, watched closely by psychiatrists and municipal officials across the nation.

By deciding to move actively to pick up and hospitalize the homeless who are unable to care for themselves or who risk harming themselves or others, New York City has moved against the decades-long tide of freer discharge of mental patients from institutions. New York's program stresses the need for asylums and other forms of supervised shelter for the most serious cases.

Despite strong initial opposition from civil rights groups, Koch and psychiatrists at the New York City Health and Hospital Corp. think that the new approach is working.

"This may be the beginning of some realization on the part of policy-makers and the public that we need to do something, that we really need to look at the system in a really drastic way," said Dr. Luis R. Marcos, the Health and Hospital Corp.'s vice president of mental hygiene services. ". . . It may not be enough. A year from now we will probably know it is not enough, but it is a good beginning."

The patients in Bellevue's special homeless psychiatric unit are just a ripple in a great tide. Municipal officials calculate New York City's homeless population at about 40,000.

At night, about 10,000 of the homeless use shelters, others are housed in welfare hotels. Health and Hospital Corp. workers estimate that about 2,000 people live on the streets, refusing to sleep in shelters.

Some studies estimate that between 40 percent and 60 percent of the homeless suffer from mental illness. Among the permanent street people, the incidence of chronic schizophrenia is believed to be still higher.

Bellevue's psychiatric emergency room, and the emergency rooms of other municipal hospitals, are always crowded. Seriously ill patients being treated and fed sometimes are handcuffed for days awaiting beds or waiting for the effects of crack, a potent form of cocaine, and other drugs to wear off.

When municipal hospital emergency rooms become too overcrowded, an arrangement city and state officials have labeled the "tripwire" is activated. Municipal hospital administrators bargain with state hospital personnel over how many of the day's patients can receive acute short-term care in some state hospital. It is not unusual for a patient to travel by ambulance to a hospital far from the city.

On Bellevue's 18 West Adult Psychiatric Unit, both patients and staff have shared a learning experience. Some staff members were apprehensive when the first group of homeless, rounded up from the streets, arrived for treatment.

"There were all these questions and we didn't know what to expect," said Jo Anne Frohock, head of activity therapies. "I think there has been a real sort of spirit of adventure: 'Let's try it. Let's see what works.' "

Many of the homeless were extremely hostile and had retreated to early stages of childhood, such as toddlers playing parallel to each other, but separately, in a sandbox. Staff members tried at first to get the homeless out of bed for a meeting.

"We met a lot of resistance, and even when we did encourage them to come out, they were isolated. They wanted to sit by themselves," said Leroy Edwards, a nursing supervisor. "They did not want to communicate with each other. You have to give them time."

One man was so silent for so long that staff members thought he was mute. "He was refusing everything. He was not saying anything," Edwards recalled.

A woman patient was just the opposite. "I had a lot of patients swear at me, but this was done really professionally. After you accept that type of language and behavior without getting excited and not getting upset, you continue to try to be therapeutic."

Finally, there was a breakthrough. Later, the patient came back to Edwards and told him, "Thanks for understanding."

To coax the homeless to activity therapy, staff members set up bingo games but saw that everyone got prizes. Activity therapists also found the homeless men and women concentrate better with background music, particularly new wave music. But getting the patients to work with each other on projects remains difficult.

"You have to respect their boundaries, because they wouldn't have gotten to the place they were, if they had good boundaries," said Frohock. "They are very fragile. When they need to back off, you have to let them have that kind of space."

Some patients, despite all the effort, remain firmly rooted in their delusions. One woman is convinced she is a federal marshal.

"We're getting results, again not in every case, but the general trend is that the patients are improving," Nardacci said. "The patients are developing, if not a limited insight, at least the ability to trust and to comply with our recommendations. We're certainly accomplishing something."

A day on the unit begins with breakfast at 7 a.m. Patients often are reluctant to shower, brush their teeth or comb their hair and have to be taught those basic skills again. After medication, they are encouraged to attend activities groups, including painting, crafts, dance therapy and other skills designed to stress self-esteem and socialization. After lunch and medication, the patients meet with physicians and resume activities. After more medication and dinner, most of the evening is spent watching television.

Even following such a simple, structured routine can be difficult for people who spend months on the streets.

"I think we are finding it is taking a lot of our patients a lot longer {than expected} to get better," said Harriet Ruschmeyer, the unit's social work supervisor.

"Working here, you need a lot of patience," said Nancy Loo, a nurse on the unit. "They need a lot of tender loving care and teaching."