WASHINGTON, Jan. 12 - The Supreme Court struck down much of the system for sentencing criminals in federal cases today, reaffirming the principle that juries, not judges, must weigh factors that can add time to a defendant's prison term.

In a 5-to-4 ruling, the justices said that the 17-year-old federal sentencing system is invalid insofar as trial judges, rather than juries, have been ruling on the facts that go into determining time behind bars.

The ruling was hardly unexpected, since the court ruled last June that Washington State's sentencing system was invalid because it conferred too much power on judges to weight the facts: the same basic issue presented in the case that was decided today.

But today's holding, in the cases titled United States v. Booker, No. 04-104, and United States v. Fanfan, No. 04-105, is nevertheless of high interest to prosecutors, defense lawyers and defendants in federal cases.

"It has been settled throughout our history that the Constitution protects every criminal defendant 'against conviction except upon proof beyond a reasonable doubt of every fact necessary to constitute the crime with which he is charged,' " the controlling opinion by Justice John Paul Stevens said.

Federal courts impose some 1,200 sentences each week, and the solicitor general's office had predicted chaos in the federal criminal-justice system if the sentencing procedures were overturned.

But when the cases decided today were argued on Oct. 4, Justice Stevens said he was "not persuaded" that a huge problem would be created by a ruling like today's, since 97 percent of federal criminal cases are settled by plea bargains.

The sentencing system that was partly invalidated today was created to minimize wide disparities in sentences imposed for seemingly similar crimes. The Supreme Court majority said today that the statute's basic purpose was a noble one, but that judges must now regard some of its provisions as guidelines rather than rules.

Most of the act that spelled out the sentencing formulas is "perfectly valid," the Supreme Court said in a summary attached to the 124 pages of opinions. "Finally, the act without its mandatory provision and related language remains consistent with Congress's intent to avoid 'unwarranted sentencing disparities,' " the court said.

A question arising from today's ruling is what Congress will do, and when, to bring the sentencing system in line with the high court's dictates. In a separate opinion, Justice Stephen G. Breyer anticipated action on Capitol Hill. "Ours, of course, is not the last word," he wrote. "The ball now lies in Congress's court."

The president of the National Association of Criminal Defense Lawyers, Barry Scheck, issued a statement calling today's ruling "a valuable opportunity to reform the federal sentencing system so as to better distinguish among defendants and use our criminal justice resources more wisely."

"This opportunity must not be squandered," Mr. Scheck said. "Congress must not react with a 'quick fix' and miss the chance to solve a lingering and serious national problem. They need to get it right this time."

The ruling was a victory for the defendants, Freddie J. Booker and Ducan Fanfan, whose sentences will now be reviewed and probably adjusted downward.

Mr. Booker was convicted by a jury in Wisconsin of possessing and intending to distribute at least 50 grams of cocaine base. The federal guidelines recommended a sentence of 20 to 22 years, but the judge imposed a 30-year term after finding that Mr. Booker had distributed 10 times that amount of cocaine in the weeks before his arrest.

The situation was the reverse in Mr. Fanfan's case. Convicted by a jury in Maine of conspiring to possess and distribute at least 500 grams of cocaine, he was sentenced by a judge to six and a half years. The trial judge, guided by the Supreme Court ruling in the Washington State case, refused the prosecutors' request to increase the sentence to 15 to 20 years on the grounds that Mr. Fanfan had been the ringleader of a conspiracy.

Justice Stevens was joined in his controlling opinion by Justices Antonin Scalia, David H. Souter, Clarence Thomas and Ruth Bader Ginsburg. The dissenters were Chief Justice William H. Rehnquist and Justices Sandra Day O'Connor, Anthony M. Kennedy and Stephen G. Breyer. As in the Washington State case, the lineups on both sides were somewhat unusual for their conservative-liberal agreements.

The dissenters maintained, unsuccessfully, that "history does not support a 'right to jury trial' in respect to sentencing facts," despite the bedrock right to a jury trial to determine guilt or innocence.