The Supreme Court said yesterday that states have the right to prevent family members from taking permanently unconscious patients off life-support systems if there is no "clear and convincing" proof that the patients want the treatment ended.

Confronting for the first time the emotional issue of "the right to die," the court, in a 5 to 4 decision, blocked the parents of Nancy Cruzan, a 32-year-old Missouri woman in a "persistent vegetative state" since a 1983 automobile accident, from stopping artificially administered food and water that keeps their daughter alive.

In examining what Chief Justice William H. Rehnquist described as a "perplexing question with unusually strong moral and ethical overtones," the court also suggested that those competent to express their wishes do enjoy a constitutional right to refuse life-sustaining medical treatment.

And in a concurring opinion, Justice Sandra Day O'Connor said that if people appoint relatives or friends to make decisions about medical treatment in the event they become incompetent, states "may well be constitutionally required" to defer to the wishes of such "surrogate decisionmakers."

The Cruzans argued that they were convinced that their once-active and independent daughter would not have wanted to continue living, but the Missouri Supreme Court had upheld the efforts of state officials to stop them from taking any action.

"The choice between life and death is a deeply personal decision of obvious and overwhelming finality," said Rehnquist, joined by Justices Byron R. White, O'Connor, Antonin Scalia and Anthony M. Kennedy. "We believe Missouri may legitimately seek to safeguard the personal element of this choice" by requiring strong evidence of a patient's wishes before allowing termination of life-sustaining treatment.

The ruling prompted two emotional dissents -- one by Justice William J. Brennan Jr., joined by Justices Thurgood Marshall and Harry A. Blackmun, and another by Justice John Paul Stevens.

Brennan accused the court of consigning Cruzan, against her will, to a "twilight zone" in which she may be forced to live as "a passive prisoner of medical technology" for perhaps another 30 years.

"Nancy Cruzan is entitled to choose to die with dignity," Brennan said.

Stevens said the decision "permits the state's abstract, undifferentiated interest in the preservation of life to overwhelm the best interests of Nancy Beth Cruzan, interests, which would, according to an undisputed finding, be served by allowing her guardians to exercise her constitutional right to discontinue medical treatment."

The ruling was greeted with dismay by medical organizations, many of which had filed friend of the court briefs arguing that society has long recognized a right to refuse medical treatment and that family members should be allowed to exercise that right for patients who cannot express their wishes themselves.

"It's this kind of decision that engenders the fear our patients frequently express that they will lose control of the decision-making" process about their medical care, said Nancy Dickey, a family physician and trustee of the American Medical Association.

But disability-rights and antiabortion groups hailed the ruling as a victory for patients who are incapable of expressing their own wishes and a protection against forced euthanasia.

The United Handicapped Federation said the decision affirms "that the degree of one's disability should not be the measure of the worth of one's life."

The Bush administration had also filed a friend of the court brief supporting Missouri, arguing that states should have the right to ensure that incompetent patients are protected against erroneous decisions to remove life-sustaining treatment.

In upholding that view yesterday, Rehnquist said the "logic" of the court's previous cases suggested that a competent patient's constitutional right to "liberty" would be "implicate{d}" by the forced administration of life-sustaining medical treatment, including artifically administered food and water.

For "purposes of this case," Rehnquist said, "we assume that the United States Constitution would grant a competent person a constitutionally protected right to refuse lifesaving hydration and nutrition."

The Cruzans authorized surgery to insert a feeding tube for their daughter three weeks after her accident, when there was still hope for recovery. Four years later they sought to have the feeding stopped, and a guardian appointed to represent Nancy's interests agreed with them that, if competent, she would want the food and water withdrawn.

But Rehnquist said that in such a situation, the state's interest in protecting the life of its citizens means it does not have to defer to the views of close family members about what their relative would want.

Although there is "no doubt . . . that Nancy Cruzan's mother and father are loving and caring people," he said, the Constitution does not require the state to leave the decision in such cases to family members who "substitute" their judgment for that of the incompetent patient.

"Close family members may have a strong feeling -- a feeling not at all ignoble or unworthy, but not entirely disinterested, either -- that they do not wish to witness the continuation of the life of a loved one which they regard as hopeless, meaningless, and even degrading," Rehnquist said.

"But there is no automatic assurance that the view of close family members will necessarily be the same as the patient's would have been had she been confronted with the prospect of her situation while competent."

He said Missouri was therefore entitled to require proof of clear and convincing evidence before terminating an incompetent patient's life support and that the Missouri Supreme Court did not err in rejecting, as inadequate proof, testimony by one of Cruzan's housemates that Cruzan had said about a year before the accident "that she would not want to live should she face life as a 'vegetable.' "

Brennan, dissenting, said the state's general interest in preserving life cannot outweigh Cruzan's wish to end life-sustaining treatment. He said the state may therefore "constitutionally impose only those procedural requirements" that are needed to make certain Cruzan's wishes have been accurately determined.

He said the state supreme court erred in requiring clear and convincing evidence, rather than some lower level of proof, and in failing to consider relevant statements Cruzan had made about her views on continuing life under such conditions.

By showing "disdain" for Cruzan's own statements and excluding her family members from deciding what their daughter would have wanted, Brennan said, Missouri and the U.S. Supreme Court "have discarded evidence of her will, ignored her values, and deprived her of the right to a decision as closely approximating her own choice as humanly possible."

In a concurring opinion, Scalia said the court should have announced, "clearly and promptly, that the federal courts have no business in this field" and that such issues should be decided by state legislatures.

Answers to "the difficult, indeed agonizing, questions that are presented by the constantly increasing power of science to keep the human body alive for longer than any reasonable person would want to inhabit it," Scalia said, are not "known to the nine Justices of this Court any better than they are known to nine people picked at random from the Kansas City telephone directory."

Joyce and Joe Cruzan yesterday issued a statement through their lawyer reiterating that they want their daughter to die.

"Since our initial request in May 1987 to the state hospital to discontinue treatment and allow Nancy the dignity of death our goal has never wavered, nor does it now," they said.

Staff writer Susan Okie contributed to this report.