In 1984, Congress established the U.S. Sentencing Commission and charged it with writing guidelines aimed at eliminating unjustified disparities in punishment for federal crimes. Two years later, Congress created a new unjustified disparity, passing a law that treated crack cocaine offenses far more harshly than offenses involving cocaine powder.

Recent decisions by the sentencing commission and the U.S. Supreme Court will slightly reduce that disparity. Congress should finish the job by eliminating a bizarre feature of U.S. drug policy that even prohibitionists have a hard time defending.

Federal law treats crack, a smokable form of cocaine, as if it were 100 times worse than cocaine powder, a snortable form of the same drug. Five grams of crack (less than a fifth of an ounce) triggers the same five-year mandatory minimum sentence as 500 grams (more than a pound) of cocaine powder. Fifty grams of crack (less than two ounces) is enough for a 10-year sentence, which requires five kilograms (more than 11 pounds) of cocaine powder.

This 100-to-1 ratio, which the sentencing commission copied in setting penalties for amounts of crack below, between and above the mandatory minimum cutoffs, undermined an important goal of the sentencing guidelines, which were supposed to distinguish between major traffickers and small-time dealers. Under the penalties created by Congress, a retailer who converts a few grams of cocaine powder into crack, or a user possessing as few as 10 doses, can get a longer sentence than a wholesaler carrying a much larger quantity of powder.

Overall, sentences for offenses involving crack are three to six times longer than sentences for offenses involving the same amount of cocaine powder. This injustice has fallen disproportionately on blacks, who account for more than four-fifths of federal crack offenders but only a quarter or so of cocaine powder offenders.

The sentencing commission, having concluded that the 100-to-1 gap between crack and cocaine powder has no scientific basis, has been urging Congress to address the disparity since 1995. It also has tried to shrink the gap on its own, only to be overruled by Congress.

In May, however, the commission sent Congress an amendment that reduces the disparity, and this time, Congress did not interfere. Last week, the commission unanimously voted to make the change retroactive, meaning that 19,500 crack offenders currently in federal prison will be eligible for resentencing. On average, the commission estimates, they could receive a sentence reduction of 27 months, from 12.7 years to 10.4 years.

The day before the commission's decision on retroactivity, the Supreme Court ruled that judges may impose sentences below the range recommended by the federal guidelines based on their disagreement with the differential treatment of crack and cocaine powder. The case involved a crack dealer who was given a sentence of 15 years instead of the 19 to 22.5 years indicated by the guidelines.

In another decision issued the same day, the Court emphasized that appeals courts should review sentences that depart from the guidelines "under a deferential abuse-of-discretion standard." Both rulings elaborated on a 2005 decision in which the court made the sentencing guidelines merely advisory after concluding that mandatory guidelines required judges to determine facts that automatically triggered longer sentences, thereby violating the Sixth Amendment right to trial by jury.

But the sentencing commission and newly empowered judges can do only so much to address the irrational, unjust disparity between crack and cocaine powder. Only Congress can change the mandatory minimum sentences required by statute.

Members of Congress have proposed bills that would shrink or eliminate the gap between crack and cocaine powder, but some want to do so partly by increasing the penalties for powder possession. Since there's no reason to believe cocaine sentences are insufficiently severe, this approach is a cop-out. Two decades after fear of a new drug fad drove Congress to establish draconian crack sentences, legislators should not let fear of seeming soft on drugs stop them from correcting that mistake.