ASHINGTON, March 8 — In a decision that could prove highly favorable for many criminal defendants, the Supreme Court on Monday made it more difficult for prosecutors to introduce statements at trial from absent witnesses who are not available for cross-examination by the defense.

The court's 9-to-0 decision addressed a situation that typically arises when a witness who has given a statement to the police later claims a privilege against testifying at trial, or when a co-defendant who has pleaded guilty has made incriminating statements during plea negotiations.

The basis for the court's decision was the Sixth Amendment's Confrontation Clause, which gives a criminal defendant the right "to be confronted with the witnesses against him." In an opinion by Justice Antonin Scalia, the court overturned a 24-year-old precedent under which a statement from a witness who was not available for cross-examination could nonetheless be used at trial if the judge found it to be reliable.

Justice Scalia said the requirement of reliability was too subjective, amorphous and "malleable" to comport with the intent of the Constitution's framers, for whom the right to cross-examine adverse witnesses was essential. The right was firmly established in English common law at the time the Constitution was adopted, he noted.

"Admitting statements deemed reliable by a judge is fundamentally at odds with the right of confrontation," Justice Scalia said, adding: "Dispensing with confrontation because testimony is obviously reliable is akin to dispensing with jury trial because a defendant is obviously guilty. This is not what the Sixth Amendment prescribes."

The decision, Crawford v. Washington, No. 02-9410, overturned an assault conviction that had been upheld by the Washington Supreme Court. Michael D. Crawford stabbed a man who, he believed, had tried to rape his wife. His defense was self-defense, and the question at trial was whether the victim was reaching for a weapon when he was stabbed. Mr. Crawford's wife, Sylvia, who was present at the time, had given the police a statement suggesting that there was no weapon. Invoking the marital privilege, she did not testify at her husband's trial, but the judge permitted the prosecution to introduce her taped statement.

Under the rule the Supreme Court established Monday, the prosecution may introduce statements only from absent witnesses who had previously been cross-examined by the defense, such as at a deposition or at a previous trial. So the rule will not hamper prosecutors in putting on evidence at a new trial following a mistrial, for example.

One criminal law expert, Prof. Richard D. Friedman of the University of Michigan Law School, said the decision was a "wonderful development" that could bring significant changes in the conduct of criminal trials. If prosecutors have any doubt about the ultimate availability of their witnesses, they will have to give the defense a chance at cross-examination before trial, he said.

"The court is saying that the Confrontation Clause is not a matter of weighing and judging, but that it means what it says," Professor Friedman said.

While all nine justices voted to overturn Mr. Crawford's conviction, Chief Justice William H. Rehnquist and Justice Sandra Day O'Connor did not sign Justice Scalia's opinion establishing the new rule. They said the statement in the Washington case did not even meet the 1980 rule's test of reliability, so there was no occasion to overrule that decision, Ohio v. Roberts.

These were other developments Monday:

Guilty Pleas

Ruling this time for the prosecution, the court unanimously overturned a decision of the Iowa Supreme Court in the case of a defendant who pleaded guilty after waiving his right to a lawyer. The Iowa court said the judge could not accept such a plea without first giving the defendant two pieces of advice: that there was a "risk that a viable defense will be overlooked," and that the defendant would be deprived of an independent opinion on whether the guilty plea was wise.

While the judge does have to make certain that a guilty plea is voluntary, this particular formula is not constitutionally required, Justice Ruth Bader Ginsburg wrote for the court in Iowa v. Tovar, No. 02-1541.

Boy Scouts

Without comment, the court refused to hear a challenge by the Boy Scouts to Connecticut's exclusion of the organization as a beneficiary of the annual state employees' charitable fund drive. The state concluded that the Scouts' policy against gay leaders violated the fund drive's nondiscrimination rule. The case was Boy Scouts of America v. Wyman, No. 03-956.