If upheld on appeal, the decision could jeopardize other drug arrests, and force police departments to monitor the patterns of stops and searches by their officers.

''This is an unusual and perhaps pathbreaking decision,'' said law professor Samuel R. Gross of the University of Michigan, who has written about racial profiling.

The Worcester district attorney's office plans to appeal Friday's ruling from McCann, who will decide today whether to release Andres Lora of New York City while the appeal is heard. The charges against Lora have not been dismissed, and he could still face a minimum sentence of 15 years if convicted of cocaine trafficking. But the case against him rests entirely on evidence that has now been thrown out, the cocaine found by troopers in the trunk and his admissions to them that he planned to sell it.

According to court records, at about 9 p.m. on Dec. 21, 2001, Trooper Brendan Shugrue stopped the Pontiac Grand Am that Lora was a passenger in, on Interstate 290 in Auburn, just south of Worcester. Shugrue, who had been parked on an offramp, followed the Pontiac, which stayed within the speed limit, made no erratic movements, and showed no equipment problems. The Pontiac, however, stayed in the left lane, which is illegal in Massachusetts except when passing or preparing for a left turn. The little-known law does not specify how long a driver may remain in the left lane, but after a mile and a half, Shugrue pulled over the car.

The trooper testified that he had seen that the men in the car were dark-skinned, but that that was not the reason for the stop.

''You put the cruiser lights on a few seconds after you observed the color of their skin,'' defense attorney Michael H. Erlich asked Shugrue, according to the records. ''Is that what you just testified to?''

''To twist my words, yes,'' Shugrue replied.

Neither of the men in the car spoke much English, but the driver, who is not named in the court record, volunteered to Shugrue that his license was suspended, and that the passenger, Lora, owned the car. The trooper testified that he took the driver to his patrol car, intending to let the men leave with Lora driving. Then he saw Lora, still in the Pontiac, talking on a cellphone, and then opening the car door, as though he might be planning to get out of the car. The trooper approached the car again, and Lora closed the door.

The trooper shined his flashlight into the car and saw a sealed plastic bag on the driver-side floor, containing what he thought was cocaine. He called for backup, and when Trooper William Pinkes arrived, they searched the trunk, finding a larger package wrapped in cellophane, next to the spare tire. It contained about 1,000 grams, or 2.2 pounds, of cocaine.

Lora, 53, who is a Dominican immigrant, was arrested and taken to the State Police barracks. The officers said he admitted that he had bought the cocaine in the Bronx for $26,000, and intended to sell it in Worcester for $30,000.

In asking the judge to exclude the cocaine and Lora's statements as evidence, Erlich obtained copies of all tickets written by Shugrue during a six-month period, and Pinkes for two months. According to the records, Shugrue had searched the cars of 5.1 percent of the white motorists he ticketed for moving violations in Auburn and Worcester, but 33.2 percent of minorities' cars. Pinkes had searched 1.6 percent of the white motorists, but 69.7 percent of minorities. Those disparities are far greater than for state troopers generally, according to a series of Boston Globe articles that the defense submitted as evidence.

In his ruling, Judge McCann found, first, that Shugrue was within his discretion to stop the car, because the driver was violating the left-lane prohibition.

But regarding the search, McCann agreed with the defense argument that the troopers' pattern in previous traffic stops was relevant. He was applying to criminal law the US Constitution's Equal Protection Clause, which is usually restricted to civil law. It is routine in civil cases alleging employment discrimination, for example, for judges to allow statistical evidence of discrimination.

The unrebutted evidence of the trooper's pattern of stops and searches, McCann wrote, ''leaves no reasonable conclusion but that Shugrue stopped the motor vehicle in which the defendant was a passenger because of the race of the operator and the race of the defendant.'' After excluding the search as evidence, McCann also threw out the statements that resulted from it. A State Police spokeswoman declined to comment on the ruling. The assistant district attorney, James R. Lemire, did not return a telephone call from the Globe, nor did the Worcester district attorney, John J. Conte.

John Reinstein, legal director for the ACLU of Massachusetts, praised the decision: ''We applaud Judge McCann for his willingness to enforce the constitutional guarantee of equal protection.''

Erlich, the defense attorney, acknowleged that the decision might be hard for some people to stomach. His client did have 1,000 grams of cocaine in his trunk.

''But the Equal Protection Clause does not focus on what was found, or how much was found, but on the process in which it was found,'' Erlich said.

The case appears to break significant ground. Professor Gross, who has researched racial profiling, said he knew of only one similar case, in New Jersey. Judge McCann, in his ruling, said the issue has not been addressed by any appeals court in Massachusetts.

Gross said that courts generally are increasingly skeptical of any use of race by police in deciding whom to stop and search.

''It's going to be a very brave attorney for the government who would say to a judge, `We stopped this person in part because he's an African-American in a white neighborhood and that's suspicious,' '' Gross said. ''I don't think those words will ever be said again in court, but 20 years ago that was common.''