Medical miracles take many forms, but few are as vivid and immediate as this: As the tiny blood vessels were sutured back together, the patient's hand flushed from porcelain to pink. The delicate tendons of the palm revived, and the skin's granite glaze began to soften.

The man's fortunes had taken a remarkable turn. So, too, had those of Dr. Maria Siemionow, a surgical resident assisting in the operation.

"That you could restore to people a part of themselves that had been lost, and actually see it become vital again, was miraculous to me," said Dr. Siemionow, a native of Poland who trained in Finland and the United States. "I have never forgotten that day."

Thirty years later, microsurgery is a commonplace marvel, and as director of plastic surgery research at the Cleveland Clinic, Dr. Siemionow, 55, is a leading practitioner.

But the career that began in a Helsinki hospital has brought her, and her profession, to an extraordinary moment. A team led by Dr. Siemionow is planning to undertake what may be the most shocking medical procedure to occur in decades: a face transplant.

After years of heated scientific debate over ethics and technical feasibility, the Cleveland Clinic last fall became the first institution to approve this novel surgery. Already Dr. Siemionow's group is searching for its first patient.

An amateur photographer - portraits of faces, mostly - with a talkative, almost merry demeanor, Dr. Siemionow is not the sort one expects to find center stage in a medical danse macabre. But this is no ordinary procedure, and she is no ordinary scientist.

"This is the single most important area of reconstructive research, and she is carrying the torch," said Dr. L. Scott Levin, chief of plastic and reconstructive surgery at the Duke University Medical Center.

"She is incredibly humble but internationally respected, and she has a powerful moral compass. She really wants to help these patients," Dr. Levin said.

Dr. Siemionow was raised in Poznan, a city midway between Warsaw and Berlin. Poznan is home to a large medical university that bears a rich academic tradition, with roots in the Renaissance. She graduated there in 1974.

"Even in my high school you were taught that medicine is a humanistic practice," she said. "Physicians are a part of their patients' lives, and there must be an intimate bond."

There are few areas of medicine in greater need of a humanistic perspective. No one knows exactly how many people live with facial disfigurement caused by burns, trauma, disease or birth defects. But Dr. Siemionow has seen too many cases already - the woman whose fianc�e left after she was burned in a car accident, the lonely shut-in whose face was lost to cancer. For these patients, there is little doctors can do to restore normal appearance.

Plastic surgeons usually "resurface" the damaged face with skin from the victim's back, buttocks or thighs. Patients may need as many as 50 operations to regain even limited function, and the results are mixed at best.

Normal facial expression, the raised eyebrows and lopsided grins so essential to social interaction, is impossible. Often the structurally complex eyelids and mouth cannot even be made to open or close properly. Even after dozens of operations, many disfigured patients must feed themselves through tubes.

And the aesthetic outcome, often likened to a mask or a living quilt, can be so unsettling that some rarely leave their homes. Chronic depression is not uncommon.

"When you mention a face transplant, people think you are talking about vanity, that someone healthy is going to be walking around with someone else's face," Dr. Siemionow said. "But within the surgical community, we perceive it as a step forward for these traumatized patients."

The procedure has been a theoretical possibility at least since 1999, when surgeons at the University of Louisville performed the nation's first hand transplant. That operation has been duplicated some two dozen times now, and the experience has given surgeons like Dr. Siemionow the courage - hubris, critics say - to think the unthinkable.

"Have you ever seen someone with severe facial disfiguration? Sometimes I have to force myself to look in a patient's eyes," said Dr. John Barker, director of plastic surgery research at the University of Louisville. "We are social animals, and the face is important to who we are as human beings."

(With scientists at the Utrecht University, Dr. Barker and colleagues at the University of Louisville also are seeking approval for an experimental face transplant to be performed in the Netherlands. Proposals by surgeons in Britain and France have been denied.)

The medical challenges to face transplantation are formidable. As Dr. Siemionow envisions it, the series of operations will require rotating teams of specialists who may be deployed in more than one operating theater. The face to be transplanted will be removed, or "degloved," from a cadaver; it will most likely include the epidermis, along with the underlying fat, nerves and blood vessels, but no musculature.

Surgeons also will remove the patient's own damaged facial tissue, then reattach the clamped blood vessels and nerves to the transplanted face. The procedures will take 15 hours, perhaps longer.

The months following may be even more harrowing. Patients receiving transplanted organs must take a lifelong regimen of drugs to suppress their own immune systems and prevent rejection. The drugs are expensive, often $1,000 per month, and the regimen does not always work. But even when it does, long-term immunosuppression increases the risk of developing life-threatening infections and cancer.

For every transplant patient, then, doctors must weigh the necessity for a new organ against the possibility of rejection and a shortened life.

Foreign skin provokes a powerful immune response from the body, more so than transplanted livers or kidneys. So despite reams of research, the risks to Dr. Siemionow's first patients are a cipher, the surgery a step into the void.

While many researchers hail her proposal as a brave leap forward, critics describe it as an alarming case of scientific overreach.

Many medical ethicists believe there are still too many unanswered questions, especially for a procedure that is not lifesaving, only life enhancing. What are the patient's prospects if the new face is rejected? What are the psychological ramifications for the recipient's family, and the donor's, if the recipient actually comes to resemble the donor?

That is unlikely, but not impossible: though the skulls will never match, there may be some resemblance if the donor's underlying facial musculature also is transplanted.

"This idea needs more evaluation. What we do know either can't be quantified or the risks clearly outweigh the benefits," said Karen Maschke, the associate for ethics and science policy at the Hastings Center, a bioethics research institute in Garrison, N.Y. "Look, a lot of science is boosterism.

"People always think they're going to be cured by new treatments and life will be normal again, but that's usually not the case."

Dr. Siemionow disputes the notion that facially disfigured patients should not be allowed to decide the risks, asking, "How can people who are normal decide for burn victims 'This is not right for you'?"

In a series of innovative experiments in laboratory rats, her team has managed to induce long-term tolerance to hind-leg transplants with a drug regimen lasting only seven days. If similar results can be achieved in humans (many previous efforts along these lines have failed), the advance will alter the calculus behind transplantations, making them feasible for a much greater number of patients, including those with facial disfigurements.

But that day may never come, and for now, the real battle may not even be scientific. Already, Hollywood, that redoubt of medical humanists, has sunk its teeth into the notion that faces someday may be exchanged.

From the moment Dr. Siemionow first proposed this surgery, she has been hearing about "Face/Off," the 1997 movie starring John Travolta and Nicolas Cage as an F.B.I. agent and a criminal mastermind whose mugs are surgically swapped.

One night, before the first review of her proposal by colleagues at the Cleveland Clinic, she rented the movie to gauge the public's potential reaction to the operation.

"It was O.K., if you like Travolta," she shrugged. "But it was just science fiction."