THE SUPREME COURT dropped a bombshell on the world of criminal sentencing last week. The case, Blakely v. Washington, is the latest and most dramatic in a line of cases in which an ideologically eclectic five-member majority has sought to breathe new life into the right to trial by jury. The cause seems noble. But this line of cases has been a Pandora's box, opened now by the Blakely decision. The decision casts grave and unwarranted constitutional doubt on sentencing regimes around the country, including federal sentencing guidelines, that have been designed to make punishments more predictable and more evenly applied.

The principle, first articulated four years ago in Apprendi v. New Jersey, seems simple and attractive from a civil libertarian point of view: "Other than the fact of a prior conviction, any fact that increases the penalty for a crime beyond the prescribed statutory maximum must be submitted to a jury and proved beyond a reasonable doubt." A fact that pushes a sentence beyond the statutory maximum for a particular crime, the theory goes, is really an element of a different, more serious crime -- and such elements, under the Constitution, must be proven to the jury. But the court has never been clear about what facts count as elements and what facts count as mere factors in sentencing, which traditionally can be weighed by judges.

Now, in Blakely, the court has answered this question, and its answer is stunningly disruptive. Ralph H. Blakely Jr. was not, in fact, sentenced beyond the statutory maximum that his kidnapping offense could have garnered him. He was sentenced to 90 months in prison on a crime that could have netted him 10 years under Washington state law. Yet under Washington's sentencing guidelines, Mr. Blakely's presumptive sentencing range was limited to only 53 months, though the sentencing judge was permitted to exceed this range if he found exceptional circumstances -- which he did. But these circumstances, Justice Antonin Scalia held for the court, must be proven to a jury. In other words, just about any factor that increases a sentence beyond a legally prescribed expectation needs to go to a jury, even if the ultimate sentence imposed remains within the range prescribed by law.

Under this principle, Justices Sandra Day O'Connor and Stephen G. Breyer argued in passionate and lucid dissents, years of sentencing reform at the state and federal level cannot stand. For many reform laws rely on factors found by judges and probation officers after conviction to guide prison time up or down. The results of forbidding this are perverse. Legislatures will either have to end the trend toward more predictable sentencing -- thereby injecting back into the justice system the unevenness and inequity that sentencing reform was meant to address -- or prosecutors will have to list every potential sentencing factor in their indictments. This latter option would mean that highly prejudicial material now kept from juries and considered only in sentencing hearings would be put before them. The results of this reckless opinion could well be a system less fair than the one that -- in the name of the rights of the accused -- it replaces.