Marsha Cunningham was no drug dealer. But when authorities busted her boyfriend in the 1990s for selling crack and powdered cocaine, they also arrested her on a crack possession charge.

Her sentence: Fifteen years behind bars, only two less than her boyfriend got.

But Cunningham is now one of nearly 20,000 inmates convicted of crack offenses who may see their prison terms reduced under new federal guidelines intended to bring retroactive fairness to drug sentencing.

“Marsha is a really good person,” her aunt, Ruby Jones of Houston, said. “She got caught up in this behind her boyfriend.”

The sentencing guidelines went into effect yesterday — the result of a December decision by the U.S. Sentencing Commission to ease the way the system came down far harder on crack-related crimes than on those involving powdered cocaine.

Previously, a person with one gram of crack would receive the same sentence as someone with 100 grams of the powdered form of cocaine. The disparity has been decried as racially discriminatory, since four of every five crack defendants in America are black, while most powdered-cocaine convictions involve whites.

“The sentences for crack cocaine have been one of the most corrosive and unjust areas of criminal law,” the head of the federal public defender’s office for the Eastern District of Virginia, Michael Nachmanoff, said. “It’s really undermined respect for the criminal justice system, not only in the African-American community but throughout the country.”

Mr. Nachmanoff said four clients of his office were being released under the new guidelines yesterday.

About 1,600 inmates are eligible for immediate release this week, but there is no way to know exactly how many will ultimately be freed, since each prisoner has to ask for a reduction and go before a judge. The remainder of the 19,500 crack defendants will become eligible for release over the next 30 years.