BALTIMORE -- The operation was a triumph of technology and human imagination.

Laboring for nearly a full day, the largest surgical team in the history of the Johns Hopkins Hospitals accomplished what seemed like a miracle last September, successfully separating Siamese twins joined at the back of the head.

Pushing the technical limits of medicine, doctors plunged the seven-month-old boys into suspended animation to halt bleeding for almost an hour while major new veins were fashioned from the one they shared.

Six months after the operation, neither Benjamin nor Patrick Binder is fully out of danger. Each has faced a mountain of complications, and doctors said they have no idea whether either will live a normal life. Though separated, they may never walk, talk or even be able to reason clearly.

"Every day, we have a greater and greater ability to do things that are technically astonishing," said Dr. Albert Owens, president of the Johns Hopkins Hospital. "But should we? The answers keep getting more difficult, and the judgments become harder and more important."

Even five years ago, the operation on the twins would have been impossible. In five more years, such surgery could be almost routine. As medicine becomes increasingly capable of producing heroic feats, experts say, doctors will be forced more often to distinguish healing from mechanical wizardry.

Although neither child is considered normal for his age, the eventual outcome for the Binders remains unknown. Last week, each had a special titanium plate implanted in his head, and they remain in rooms at Children's Hospital. Their doctors hope to send them home to Ulm, West Germany, soon.

Members of the surgical team at Hopkins, all of whom worked without pay, said they were convinced that the surgery was necessary and that both children could recover. Many of them still are.

But the expensive procedure, which could eventually cost $1 million or more, and its questionable results raise anew questions about when and whether to perform experimental surgery.

"When you are dealing with dying children and desperate parents, society has always said we want everything done that can be done," said Dr. Arthur Caplan, director of the Center for Biomedical Ethics at the University of Minnesota. "But it's very hard to draw the line between what is clearly an experiment and what is not."

At Hopkins, where surgery on the frontiers is relatively common, that line is difficult to find. Every major participant in the Binder operation -- more than 70 doctors, nurses and technicians were in the operating room -- said the children's welfare was the only priority. None would deny the excitement of breaking new surgical ground.

"The operation demonstrated the capabilities that medical science now possesses," said Dr. Ben Carson, director of pediatric neurosurgery. "For that reason alone, it was worthwhile."

Left alone, he said, the children would have had no hope of happiness.

"Their lives would have been miserable and short," he said. "To me, that justifies the great risk we took."

Each member of the surgical team met individually with the boys' parents to discuss the surgery and its possible results. Siamese twins joined at the head had never been separated successfully, and the doctors said they wanted to make sure that the Binders were aware of the long odds.

The operation brought together dozens of physicians, nurses and assistants. A new electrical system had to be devised for the operating room to ensure continuous light and power for tools. Doctors practiced for weeks on dolls joined at the head with Velcro. Using new computer techniques, precise replicas of the boys' skulls were created so surgeons could cover parts of the actual skulls with protective titanium plates. That was deferred until last week.

No formal review board met to consider the surgery but, after several long discussions, physicians at Hopkins reached a consensus that the procedure was worth the risks. They also agreed to donate their time and to defray the hundreds of thousands of dollars' difference between what the parents' insurance would pay and what the operation would cost.

"These were not easy discussions," Owens said. "But the fact is this hospital is a place of healing and discovery. They go hand in hand here, and they can't be separated."

New drug therapies must pass several levels of review, in the hospital and at the Food and Drug Administration, before being given to patients. That is not so for surgery.

Some doctors have said a more formal procedure should be devised before opting for such experimental solutions. Hopkins is recruiting a doctor who specializes in ethics to help advise physicians about such increasingly difficult decisions.

Many surgeons have said no new federal regulation is needed, particularly in a field where immediate intervention is often necessary.

"I am willing to take any reasonable risk to separate Siamese twins because of the grotesque future they face," said Surgeon General C. Everett Koop, who has separated three pairs. "These are always terrible ethical decisions, but they have to be left in the hands of the surgeons who can determine what the best possible result will be."

In one famous operation, performed in 1976 while Koop was chief of surgery at Children's Hospital in Philadelphia, he separated babies sharing a heart. One died.

"For one to live, one had to die," Koop said. "The heart really belonged to one child and not the other, but it was a very difficult experience for me."

Surgeons involved in the Binder operations quickly ruled out attempting to save only one child. But they also ruled out approaching the surgery strictly as experimental research.

"We knew we were breaking new ground, and that was clearly exciting," said Dr. Mark C. Rogers, head of anesthesiology and critical care medicine, who led the surgical team. "But at no time did we ever forget we were dealing with two human lives.

"I wish I could explain what it is like when people come to you and you might have their only hope," he continued. "It's not fun. It's a burden and a responsibility. We could make no mistakes. There was no margin for error."

Rogers said his only second thoughts have been about the cost of the operation.

"We have a real conflict," he said. "Yes, that money could have been spent in other ways to benefit many. But advances in medicine have to be made at a sacrifice.

"Our job is to help the patient, not to decide how society should parcel out its resources. We did everything in our power to help the Binder children, and you can't have regrets about that."