In an impassioned 177-page decision, the judge, William G. Young, described a system in which prosecutors used various strategies to reward those who pleaded guilty and to impose exceptionally harsh sentences on those who chose to stand trial and then lost.

''The focus of our entire criminal justice system has shifted away from trials and juries and adjudication to a massive system of sentence bargaining that is heavily rigged against the accused citizen,'' Judge Young, the chief judge of the Federal District Court in Boston, wrote in a decision reconsidering the sentences he had imposed on two defendants.

Judge Young acknowledged that similar challenges to the sentencing guidelines had been rejected by all of the appeals courts that had considered them. The Supreme Court upheld the constitutionality of the guidelines themselves in 1989, but he said more recent Supreme Court cases supported his analysis.

The Supreme Court is now considering a similar issue in the context of Washington State's sentencing laws.

Judge Young said that about 97 percent of federal criminal convictions nationwide were the result of plea bargains. Defendants accused of federal crimes in Massachusetts who insist on a trial, he added, face sentences six times as long as those they would receive after a guilty plea and an agreement to cooperate with prosecutors.

Judge Young cited a litany of methods used by prosecutors to encourage defendants to plead guilty under federal sentencing guidelines issued by a commission created by Congress in 1984. They include, he said, ''charge bargaining,'' in which prosecutors drop selected charges in exchange for a plea, and ''fact bargaining,'' in which prosecutors turn a blind eye to evidence, typically guns or drugs, that would require a harsher sentence.

Michael J. Sullivan, the United States attorney in Boston, said Judge Young's accusations were not accurate.

''We don't do charge or fact bargaining,'' Mr. Sullivan said. ''You charge based on what the evidence would support and what the law would allow. You don't overcharge. You charge based on the available evidence.''

Judge Young's opinion is representative of many federal judges' dissatisfaction with their limited role in criminal sentencing, said Frank O. Bowman, a law professor at Indiana University and an author of a treatise on the subject. The decision is, he said, ''a long cry of anguish about the state of federal sentencing, which, while perhaps a little overheated, is an excellent summary of the current state of things.''

Judge Young relied on two recent Supreme Court cases to hold the sentencing guidelines unconstitutional. Though the later of the cases was decided in 2002, he wrote that he had not fully appreciated their significance until he read an article by William J. Trach, then a student at Harvard Law School, in the February Harvard Law Review.

The two cases, Judge Young reasoned, require that all facts that increase the punishment to which a defendant is exposed be proved to a jury. But sentencing hearings take place before only judges, and the rules of evidence do not apply.

''Courts today,'' he wrote, ''must base their conclusions on a mishmash of data including blatantly self-serving hearsay'' presented by prosecutors. The data, he wrote, often include information about crimes with which the defendants have never been charged and even crimes of which they have been acquitted.

Because the sentencing guidelines allow judges to increase sentences based on such material, he continued, the procedure can be unconstitutional in given cases.

Judge Young's decision concerned the sentences of five defendants convicted on drug charges.

He had sentenced Richard Green to 20 years in March and William Olivero to four years in June. On reflection, he wrote, both sentences were too long. In Mr. Green's case, he said he had been wrong to rely on evidence about guns and other drugs that the jury had not heard. The right sentence, he wrote, was four years.

In Mr. Olivero's case, Judge Young said, prosecutors engaged in ''starkly illegal fact bargaining.'' When Mr. Olivero was offered a plea bargain, the judge wrote, the charges had no reference to a gun; after trial, prosecutors relied on the gun for an enhanced sentence. The right sentence, he concluded, was 16 months.

Both cases are on appeal, which means that Judge Young no longer has jurisdiction. He asked the appeals court to return the cases to him for resentencings.

He sentenced a third defendant, Jason Pacheco, to 12 years, saying the result would have been the same before his new analysis.

He said he would issue decisions on the other two defendants soon.

Copyright 2004 The New York Times Company