MILWAUKEE, FEB. 15 -- As the relatives of his victims quietly wept, a jury decided today that confessed serial killer Jeffrey L. Dahmer was legally sane when he murdered and dismembered 15 boys and men in one of the nation's most grotesque killing sprees.

Dahmer, who faces mandatory life imprisonment for each of the murders, will be sentenced Monday. Wisconsin does not have capital punishment.

The dramatic end to the case that shocked and horrified this city and the country came as Milwaukee County Circuit Judge Laurence C. Gram Jr. read 15 separate verdicts that Dahmer did not suffer from mental disease at the time of each of the killings. When Gram read the 15th verdict, assuring that Dahmer would be imprisoned and not committed to a state mental institution, cheers and applause erupted in the part of the courtroom reserved for relatives of Dahmer's victims.

Two of the 12 jurors disagreed with the verdicts, but the votes of only 10 were necessary for a ruling in the unusual sanity trial.

Dahmer pleaded guilty to the murders last month. He also pleaded not guilty by reason of insanity. To find him insane, 10 of the jurors had to agree that he suffered from a mental disease that prevented him either from knowing right from wrong or from being able to control his actions.

While the trial centered on the question of whether Dahmer could control himself, the jury never got that far, ruling that he did not suffer from a mental disease.

"This met my fondest hope," Milwaukee County District Attorney E. Michael McCann said of the verdict. "The tragedy here is I think he could have stopped at any time."

Dahmer's defense attorney, Gerald P. Boyle, said he warned the 31-year-old former chocolate factory worker earlier today that he would probably lose the insanity defense. After the verdict, Boyle said, Dahmer told him, " 'Thanks for trying.' "

"I know we have brought some peace of mind to him," Boyle said. "Now he knows he was sick. He wasn't so sick as to give him a defense."

One of the dissenting jurors, Russell P. Fenstermaker, 46, said that while he thought that Dahmer suffered from a mental disease he did not disagree with the substance of the final verdicts because he also believed Dahmer could control himself. The other dissenting juror, Clare M. Horvath, did not speak with reporters following the trial.

Another juror, Elba C. Duggins, said the verdicts were reached without heated argument. "We all agreed there was a problem {with Dahmer}," she said. "Whether we interpreted it as a disease or disorder is what we discussed."

Asked how she and the others would deal with the trial ordeal and its days of extraordinarily grisly testimony, Duggins said, "I already dealt with it partially by leaving the room and crying."

Juror Karl W. Stahle, 65, a retired shipping clerk, said he was persuaded by McCann's emphasis that Dahmer planned the killings in advance. "I think {Dahmer} was a real con artist," Stahle said. "He could even fool the police and get away with it. I came to the conclusion he was not sick."

The relatives of Dahmer's victims praised the jury of five women and seven men who deliberated for about five hours beginning this morning.

"I am just overwhelmed by that verdict," said Therese Smith, the sister of Edward W. Smith, 28, who was killed by Dahmer in June 1990.

Dahmer sat impassively, his eyes downcast, as Gram read the 15 verdicts. His father, Lionel, and stepmother, Shari, also were in the courtroom. After the verdicts were read, the Rev. Gene Champion, a Baptist minister who has been counseling some of the relatives of the victims, spoke briefly with Dahmer's parents.

"They're victims too," he said. "They're hurting. They need help too."

Champion and others also said they hoped the verdicts by a jury that included only one black would begin to diminish the racial tension here in the wake of a murderous rampage by a gay white whose victims were mostly gay black men.

Following the verdicts, the jury met with state-appointed psychiatrists. Jurors have been offered free counseling after hearing some of the most gruesome testimony ever presented in a courtroom, including graphic accounts of how Dahmer slit open his victims, performed sex acts on their corpses and ate body parts of some of them.

Under Wisconsin law, Boyle had the burden of proving that Dahmer was insane at the time of the killings. However, the level of proof that Boyle was required to meet was not the "beyond a reasonable doubt" standard used in criminal trials, but "the greater weight of the evidence" measurement that applies to civil cases.

The key issue throughout the trial was whether Dahmer could control his bizarre impulses for sex with the dead. He acknowledged repeatedly that he knew his actions were wrong, leaving a claim that he could not control himself as the only basis for an insanity defense.

Hoping for an insanity verdict in at least one of the 15 murder counts, Boyle stressed what he called the progressive nature of Dahmer's mental illness. He said that after Dahmer killed his fifth victim, Anthony Sears, in March 1989, "We've got ourselves a very, very sick, uncontrollable young man."

"He was so impaired as he went along on his killing sprees that he could not stop," Boyle told the jury in his closing argument Friday. "He was a runaway train on a track of madness, picking up steam all the time, on and on and on, and it was only going to stop when he hit a concrete barrier or hit another train."

But McCann scoffed at the suggestion that Dahmer could not control himself. He chose his victims carefully, McCann argued, always targeting men who did not have a car because he knew that automobiles could be used to trace missing persons. Dahmer met one of his last victims in Chicago and persuaded the man to accompany him back to his apartment in Milwaukee, McCann noted.

"Do you think that was a madman . . . a wild-eyed madman?" McCann asked the jury.

"Remember, it isn't the killing he takes pleasure in, it's the sex," the prosecutor said in his final argument. Dahmer killed so he could have "a couple more days" of sexual pleasure with the bodies of his victims, McCann said. Afterward, McCann added, Dahmer dismembered the bodies not because of an uncontrollable impulse but "to get rid of the evidence."

Dahmer has confessed to the murder and dismemberment of 17 men and boys. He faces a separate murder charge in Ohio in connection with the first killing of Steven Mark Hicks near Dahmer's boyhood home in Bath, Ohio, in 1978. He has not been charged with the first murder in Wisconsin in 1987 because the body of the victim, Steven W. Tuomi, has never been found.

Dahmer's killing spree came to an end on a hot night last July when one of his intended victims, Tracy Edwards, escaped before Dahmer could drug him with powdered sleeping pills. With handcuffs dangling from one wrist, Edwards punched Dahmer, ran into the street and led police back to the second-floor apartment in a rundown neighborhood just west of downtown Milwaukee.

The discovery there of the severed heads and other body parts of 11 of Dahmer's victims traumatized this city and renewed charges of racism against the Milwaukee police. It led directly to the creation of a special commission that last fall recommended sweeping changes in police training and operating procedures.

Throughout the trial, Boyle insisted -- without disagreement from McCann -- that Dahmer's murders were not racially motivated. Instead, he said, Dahmer was attracted to a certain male body type -- slim and youthful.

The incident that most shocked the city was the discovery that last May, two months before the murderous rampage was stopped, police were called to Dahmer's apartment by reports of a naked, bleeding youth in the street. Dahmer convinced the police officers that the youth was his gay lover who had gotten drunk. The officers returned him to Dahmer's apartment.

During the trial, it was disclosed that the 14-year-old youth, Konerak Sinthasomphone, the son of Laotian immigrants, had been a victim of one of Dahmer's crude lobotomy attempts. Dahmer drilled holes in his head and poured in acid. It also was disclosed that while police were in Dahmer's living room, the body of his last victim was in the bedroom.

Two of the police officers involved were fired and a third was placed on probation. Dahmer killed Sinthasomphone shortly after police left his apartment. He also killed four others before he was discovered two months later.