Mr. Flood, who has served 10 years in prison over crimes involving crack cocaine, had been scheduled for release in 2013. Under new sentencing rules that came into effect on Tuesday, however, he is one of more than 1,800 prisoners eligible for release right away.

Mr. Flood, like tens of thousands of other prisoners, had received a far tougher sentence for a crime involving crack cocaine than he would have if the drug had been in powdered form — a disparity commonly referred to as 100 times greater for crack than for powder.

Mr. Flood’s sister, Susan Cardwell, said in an interview that Mr. Flood was scheduled for release from prison in Ashland, Ky. on Tuesday night, and would board an overnight bus to rejoin the family in Virginia. “He told me he wanted me to send him $30 for the bus ride so he could eat on the way home — anything he wanted,” she said. “After jail food for so long,” she added, she expected their first stop would be an all-you-can-eat buffet.

At the time the law was passed in the 1980s, cocaine use and crime associated with it were skyrocketing. Marc Mauer, the executive director of the Sentencing Project in Washington, said that the much of the spike had to do with initial turf wars as the drug hit the streets. Thousands of nonviolent criminals ended up with sentences that have long been criticized as extreme, and the disparity was increasingly viewed as racially tinged.

Congress addressed the issue by passing the Fair Sentencing Act of 2010, which reduced the sentencing disparity to 18 to 1. In June, the United States Sentencing Commission voted to apply the guidelines retroactively, with the new policy going into effect on Monday this week.

Over time, some 12,000 inmates could have their sentences, which average 13 years, shortened by an average of three years. Mr. Mauer noted that this still left “substantial” penalties for the crimes. “We’re not talking about a slap on the wrist here,” he said.

Michael Nachmanoff, federal public defender for the Eastern District of Virginia, said that his office has been scrambling for months to obtain court orders that will free 75 prisoners eligible for immediate release. The new rule, he said, “will save tens of millions of dollars, and hopefully allow the bureau of prisons to use those beds to house more dangerous people,” he said.

Chris Burke, a spokesman for the federal Bureau of Prisons, said, “I would say that we’re having a busy day.” With more than 1,800 prisoners eligible for immediate release, prisons have been checking records to ensure that none have complicating factors like concurrent sentences or pending charges, he said. At the bureau’s sentence computation center in Grand Prairie, Tex., he said, employees pulled an all-nighter to ensure that all of the release paperwork was processed in time.

Not everyone is pleased with the change. William Otis, a former federal prosecutor and special White House counsel under the first President George Bush, said that since recidivism is predictable, the releases would inevitably lead to more crime. “Why, when we hear in such specifics about the alleged benefits of the forthcoming releases, we do not also and simultaneously hear about the additional crime to which they will lead?” he said.

Others say the new rule does not reduce the disparity enough. Douglas A. Berman, an expert in sentencing at Ohio State University’s law school, said, “The celebration of the new reforms has to be tempered by the reality that it’s, at the most, half a loaf.”

Lawrence Garrison, who was able to shave 36 months off his prison sentence during a previous disparity reduction and was released in 2009, said, “It’s a great step,” but added: “The disparity in sentencing is still there. The racial disparity is still there.”