The judge in one of the most lurid mass child-molestation cases of the 1980s ordered a new trial for one of the defendants Friday, and ruled that the youngsters cannot testify because they have been hopelessly tainted by investigators' leading questions.

The ruling gutted the case against Cheryl Amirault LeFave, who was convicted along with her mother and brother of molesting children at the Fells Acres day-care center that the family ran.Citing new research on children's courtroom behavior, Superior Court Judge Isaac A. Borenstein said that because of prosecutors' suggestive and leading interview techniques, it is impossible to tell if the accusers, now teenagers, were telling the truth.

"Their testimony has been forever rendered unreliable," Borenstein said.

Prosecutors immediately appealed the ruling.

"Without the children, there is no case, and that is what is outrageous about this ruling," prosecutor Lynn Rooney said.

LeFave, 39, her late mother Violet Amirault, and her brother Gerald "Tooky" Amirault were all convicted of molesting about 40 children, ages 3 to 6, at the day-care center in Malden. None of the youngsters has publicly recanted any of the allegations.

On Friday, Borenstein dismissed all the charges against Amirault, who died at age 74 last September. Gerald Amirault, who was tried and convicted separately in 1986, remains in prison.

Their convictions came during a wave of mass child-molestation cases in the 1980s. Several other convictions from that period have been overturned.

Borenstein said the Fells Acres case has two tragedies: "the inability to ever know accurately and reliably whether these children were ever abused and secondly sacrificing the rights" of the accused.

As the judge read his decision, LeFave raised one trembling hand to her mouth, grasped her attorney's hand with the other, and began to cry. Several family members also wept.

"I felt humanized by a judge for the first time," LeFave said.

In his ruling, Borenstein read aloud testimony in which a child denied at least 10 times that LeFave had ever photographed her nude. Only after several interviews by police, her parents and social workers did the girl say that LeFave had abused her, the judge said.

In an April hearing, Rooney conceded that prosecution expert Susan Kelley used flawed techniques to question children. But she argued it didn't discredit the children's testimony against LeFave.

Among other lurid mass child-abuse cases of the 1980s to fall apart in the courts under similar circumstances are the McMartin preschool case in California, the Little Rascals case in North Carolina and the Margaret Kelly Michaels case in New Jersey.

In 1996, New Jersey's Supreme Court concluded in the Michaels case that investigators had conducted "coercive and highly suggestive" interviews. An appeals court ordered a hearing to determine if the children's testimony was tainted. But prosecutors dropped the case.

Victor Vieth, an attorney with the National Center for the Prosecution of Child Abuse, said the Massachusetts ruling could affect similar mass child-abuse cases.

But he said the research upon which the judge based his decision centers on the suggestibility of very young children. The overwhelming majority of child abuse victims are on average 10 years old, and research suggests that older children do not make up things based on what adults tell them, Vieth said.