The operation looks nifty in scientific drawings, makes elegant anatomic sense, and for almost two decades has been performed on thousands of patients in the belief that it would prevent strokes.

Now, a painstakingly executed international study has proven that the surgery -- called extracranial/intracranial bypass -- is worthless.

"The data are crisp. There are no ifs, ands or buts. The EC/IC bypass is not efficacious," says Dr. Murray Goldstein, director of the National Institute of Neurological and Communicative Disorders, which funded the study.

Goldstein says the trial's conclusions, which were reported last week by Dr. H.L.M. Barnett at the Eighth International Congress of Neurological Surgery in Toronto, will probably force the bypass procedure, now widely performed in the United States, Canada and Europe, to be virtually abandoned.

This marks the first time a controlled clinical trial has conclusively shown that a popular operation does not work, Goldstein says. As a result, the effectiveness of other controversial surgeries -- such as the coronary artery bypass -- could be the subject of similar scrutiny.

The EC/IC bypass was invented in 1967 as a way to increase blood flow to the brain in patients with blockages of the internal carotid artery, a major blood vessel inside the skull that is inaccessible to the scalpel. Narrowings of the internal carotid and its branches have long tantalized neurosurgeons like fruit ripening just out of reach. Angiograms -- X-rays in which dye is injected to show blood vessels -- sometimes spotlight such narrowings as the source of a patient's neurological symptoms. Yet obstructions in these vessels are difficult to approach surgically, and no operation has been clearly proven to relieve them.

The EC/IC attempts to redirect blood around such blockages.

Of the 400,000 strokes occurring in the United States each year, the majority -- about 75 percent -- are caused by blockage in one of the blood vessels supplying the brain. An artery can close down gradually, usually because of progressive atherosclerosis (hardening of the arteries), or it can be suddenly obstructed by a blood clot that travels from elsewhere in the body and lodges within its walls.

When a blocked artery deprives part of the brain of its entire blood supply, nerve cells in the area die, and a stroke -- a permanent defect in brain function -- results. But some patients with impending strokes receive advance warning. As blood supply falters, they suffer brief neurologic symptoms, called transient ischemic attacks or t.i.a.'s, due to temporary malfunction of nerve cells in the area of fluctuating blood supply. Such symptoms can include headaches, visual changes or sudden blindness in one eye, speech disturbances, weakness of part of the body or fainting spells, depending on the region of the brain affected.

Because t.i.a.'s often herald the onset of a stroke, people who have them usually undergo immediate tests to search for a blocked vessel that might be opened by surgery before the brain is damaged.

A common site of narrowing is the point where the left or right carotid artery splits to become the external and internal carotid arteries. Since these junctions lie outside the skull, one on each side of the neck, they are easy to reach. For many years, surgeons have performed an operation called endarterectomy to relieve obstructions in this area.

The EC/IC bypass seems an ingenious way of coping with narrowings of the internal carotid too high inside the skull to be treated by a standard endarterectomy. It involves removing a section of bone from a patient's skull and then, working under microscopes, hooking up the superficial temporal artery, which lies on the side of the head outside the skull, to the middle cerebral artery, a branch of the internal carotid running on the brain's surface. The idea is to shunt blood from the external carotid artery to the middle cerebral, rerouting flow around the blockage to restore nutrients to the threatened region of brain tissue.

In the decade that followed its invention, the operation was performed about 3,000 times in Europe and the United States. There are no exact figures available on how many such bypasses are being done now, but Goldstein says its popularity among neurosurgeons has continued to grow. "There's one hell of a lot of it done," he says. "It's not a difficult surgical procedure even though it goes into the brain. Any skilled neurosurgeon can do it."

Several reports from individual surgeons suggested it improved blood flow and helped prevent strokes, but they were based on small numbers and included no control group of patients with similar blockages who were observed without surgery. So, in 1977, an international trial of EC/IC bypass was begun under the direction of Dr. Barnett at the University of Western Ontario to find out whether the bypass could actually reduce the risk of stroke and death in patients who had suffered t.i.a.'s or minor strokes from obstruction of the internal carotid or middle cerebral arteries.

The study enrolled 1,377 patients at 71 medical centers, assigned them randomly to either a surgical or a medical treatment group, and followed their progress for an average of five years. Because brain surgery carries risks of complications and death, the scientists running the trial stipulated that the operation would have to reduce the risk of subsequent stroke by at least 33 percent in order to be considered superior.

The study was meticulously conducted. Patients were chosen according to strict specifications, and all participating surgeons had to agree to perform the bypass in the same way. "Not a single case was lost to follow-up," said Goldstein. "Some patients in Italy got caught in an earthquake. They the researchers sent teams out to find the patients to make sure they didn't get lost."

Data comparing the two groups were analyzed every six months to see whether either group was being harmed or whether the results were conclusive enough to end the study. Goldstein said the trial was allowed to continue even though there seemed to be no real difference in outcome between the two groups, because the researchers felt more information might change the conclusions or identify a subgroup for whom the operation was beneficial. "We couldn't convince ourselves that future data wouldn't change the story," he says.

But the final analysis showed the surgical patients had actually fared worse.

"Surgical treatment was always poorer than medical treatment," Goldstein says. Even after eliminating those strokes and deaths caused directly by the operation -- an "acceptable" 1.3 percent complication rate -- the patients who underwent surgery still suffered more strokes and fatalities than those who did not.

Detailed results of the trial will be published in The New England Journal of Medicine in the next few weeks, Goldstein says. He predicted its findings would persuade the majority of neurosurgeons in the United States and Canada to stop performing IC/EC bypass, and speculated that insurance companies might also refuse to pay for it.

"I assume that when the third-party payers see the New England Journal article," he says, "they're going to start thinking very seriously about it."