ASHINGTON, Feb. 24 — The Supreme Court overturned a Texas inmate's death sentence on Tuesday on the ground that the prosecution deliberately withheld evidence that would have made jurors less likely to impose the death penalty had they been made aware of it.

In a 7-to-2 decision, the court said that the undisclosed evidence would have allowed the defendant, Delma Banks, to discredit two of the state's most important witnesses. It would thus have allowed him to cast doubt on the prosecution's claim, essential for a death penalty under Texas law at the time, that he would pose a continuing danger to society if not executed.

In her majority opinion, Justice Ruth Bader Ginsburg directly rebuked the Texas prosecutors for concealing facts they had a legal obligation to disclose and for permitting the state's witnesses to testify untruthfully. Just as forcefully, if implicitly, the majority also rebuked the federal appeals court that in rejecting Mr. Banks's appeal in August 2002 blamed him for having failed earlier to uncover the evidence that the state had withheld.

The justices have expressed increasing concern that the lower federal courts are too cursory in reviewing death row appeals. For example, almost exactly a year ago the court voted 8 to 1 to order a new hearing for another Texas death row inmate, Thomas Miller-El. Writing for the majority then, Justice Anthony M. Kennedy said that while Congress had recently instructed the federal courts to accord greater deference to state court decisions, "deference does not imply abandonment or abdication of judicial review."

Mr. Banks, who was convicted in 1980 of killing a 16-year-old co-worker at a Texarkana steak house to steal his car, came within 10 minutes of execution last March before the Supreme Court granted a stay and agreed to hear his appeal.

In 2000, a federal district judge in Texarkana granted Mr. Banks a writ of habeas corpus, but the United States Court of Appeals for the Fifth Circuit overturned it, in part on the ground that Mr. Banks should have discovered the withheld evidence earlier and presented it to the Texas state courts.

On Tuesday, noting that the prosecutors had asserted both at trial and in state court appeals that the state had given the defense all the evidence to which it was legally entitled, Justice Ginsburg called it "appropriate for Banks to assume that his prosecutors would not stoop to improper litigation conduct to advance prospects for gaining a conviction."

She added, "It was not incumbent on Banks to prove these representations false; rather, Banks was entitled to treat the prosecutor's submissions as truthful."

Further, Justice Ginsburg said, "A rule thus declaring `prosecutor may hide, defendant must seek,' is not tenable in a system constitutionally bound to accord defendants due process."

The evidence the prosecutors withheld was their relationship with two of the state's star witnesses. One, Robert Farr, turned out to be a paid police informant, though he denied this on the stand and the prosecutors did not correct him. His testimony — that Mr. Banks offered to get him a gun with which he could commit armed robberies — was devastating to the defense because the prosecution used it to show that Mr. Banks, who had no previous criminal record, would present a future danger to society.

After Mr. Banks's new legal team tracked down Mr. Farr in 1999 in preparation for the federal habeas corpus petition, the former witness said he had "set Delma up" by pretending to want a gun for robberies that he did not really intend to commit. He said that he was afraid that the local sheriff would arrest him on drug charges if he did not agree to help convict Mr. Banks.

The second witness, Charles Cook, also gave damaging testimony and told the jury that he had not had previous discussions with the police. But in 1999, when a federal magistrate judge ordered disclosure of the files in the Bowie County district attorney's office, the lawyers found a 74-page transcript from 1980 that showed Mr. Cook being interrogated and exhaustively rehearsed in testimony that he would give at the trial a few weeks later.

In setting aside the death sentence, the court stopped short of overturning Mr. Banks's 24-year-old conviction. Now 45, he is one of the longest-serving residents of the Texas death row; Walter Bell Jr., who was sentenced to death 28 years ago, holds the record among current inmates there.

The court said Tuesday that the lower federal courts had erroneously refused to grant Mr. Banks a hearing on his claim that prosecutorial misconduct had tainted the conviction itself. So his legal team, headed by George H. Kendall, who argued the case in December, will next have a chance to show that the conviction should be set aside as well.

The justices were unanimous in the part of the opinion ordering the lower courts to consider the challenge to the conviction. Justices Clarence Thomas and Antonin Scalia dissented from the part of the opinion overturning the death sentence. Justice Thomas said that though it was "a very close question," he was not persuaded that the withheld evidence would have made a difference had it been presented to the jury.

The muted tone of the dissenting opinion was notable, as was the fact that Chief Justice William H. Rehnquist joined the majority. Clearly, the court was troubled by the facts of this case and by the appeals court's handling of it.

The case, Banks vs. Dretke, No. 02-8286, had attracted an unusual degree of support. The request for a stay of execution last March was backed by a group of retired federal judges, including William S. Sessions, a former federal district judge from Texas and former director of the F.B.I. In their brief, the judges told the court that "the integrity of the death penalty in this country" was at stake. On Tuesday, Judge Sessions called the decision "a clear victory for justice."

There were also these other decisions from the court Tuesday:

Age Discrimination

The court ruled that the federal law against age discrimination in employment was meant to protect workers whose employers think they are too old, not those who are disfavored for being too young.

Voting 6 to 3, the court rejected an age-discrimination case brought against a division of General Dynamics by a group of workers aged 40 to 50. They contended that the company's plan to eliminate future retirement health benefits for all employees who were then not yet 50 amounted to discrimination in favor of the older workers. The United States Court of Appeals for the Sixth Circuit, in Cincinnati, allowed the case to proceed under this reverse-discrimination theory.

Writing for the majority, Justice David H. Souter said the law, which gives protection against age discrimination starting at 40, had to be interpreted in light of "social history" that "emphatically reveals an understanding of age discrimination as aimed against the old." He added, "The enemy of 40 is 30, not 50."

Justices Scalia, Thomas, and Kennedy dissented. Justice Thomas said "social history" was an inappropriate basis for interpreting a statute. The case was General Dynamics Land Systems Ind. v. Cline, No. 02-1080.

Illegal Search

The court upheld a federal appeals court's decision permitting a private suit for damages against a federal agent for leading an illegal search of a Montana ranch. The search warrant described only a two-story blue house, without specifying the details the Fourth Amendment requires.

The agent was not entitled to immunity because any law enforcement agent should have recognized the defect, Justice John Paul Stevens said for the 5-to-4 majority in Groh v. Ramirez, No. 02-811. Justices Thomas, Scalia, Kennedy, and Chief Justice Rehnquist dissented.