Yates' lawyers had argued at a hearing last month before a three-judge panel of the First Court of Appeals in Houston that psychiatrist Park Dietz was wrong when he mentioned an episode of the TV show "Law & Order" involving a woman found innocent by reason of insanity for drowning her children.

After jurors found Yates guilty, attorneys in the case and jurors learned no such episode existed.

"We conclude that there is a reasonable likelihood that Dr. Dietz's false testimony could have affected the judgment of the jury," the court ruled. "We further conclude that Dr. Dietz's false testimony affected the substantial rights of appellant."

The appellate ruling returns the case for a new trial, although prosecutors said they hoped instead to successfully appeal Thursday's ruling.

"We fully intend to pursue a motion for a rehearing," said Harris County Assistant District Attorney Alan Curry, who argued the case before the appeals court. "Barring that, we'll continue to appeal to the Texas Court of Criminal Appeals. We still believe we have a good shot to prevail in appeal."

Jurors in 2002 sentenced Yates to life in prison in the 2001 deaths of three of her children. She was not tried in the deaths of the other two.

The defense's appeal cited 19 errors from her trial, but the appeals court said since the false testimony issue reversed the conviction, it was not ruling on the other matters. Among other things, Yates attorneys had claimed the Texas insanity standard is unconstitutional.

Prosecutors told the court last month there was no evidence Dietz intentionally lied and that the testimony was evoked by Yates' defense attorney during cross-examination. They also argued that Dietz's testimony wasn't material to the case and there was plenty of other testimony about Yates' plans to kill her children.

"We agree that this case does not involve the state's knowing use of perjured testimony," the appeals court said in its ruling. But the judges said prosecutors did use the testimony twice and referred to it in closing arguments.

A woman answering the telephone at Dietz's Newport Beach, Calif., office said Thursday there was no immediate comment from him or his firm. He had testified the episode aired shortly before the drownings, and other testimony during the trial had indicated that Yates watched the series.

The error came to light during the sentencing phase of the trial. State District Judge Belinda Hill refused a defense request for a mistrial but allowed the attorneys to stipulate to jurors, before they decided on Yates' punishment, that the program did not exist.

Prosecutor Joe Owmby said at the time that Dietz didn't tell him until after his closing arguments in the guilt phase of the trial that he was mistaken about the show.

"He was confused and made an error," Owmby said.

A wet and bedraggled Yates called police to her home on June 20, 2001, and showed them the bodies of her five children: Noah, 7, John, 5, Paul, 3, Luke, 2, and 6-month-old Mary. She had called them into the bathroom and drowned them one by one.

According to testimony, Yates was overwhelmed by motherhood, considered herself a bad mother, and had attempted suicide and been hospitalized for depression.

Prosecutors acknowledged she was mentally ill but argued that she could tell right from wrong and was thus not legally insane.

The case stirred debate over the legal standard for mental illness and whether postpartum depression is properly recognized and taken seriously. Women's groups had harshly criticized prosecutors for pushing for the death penalty.

Dietz is a nationally known expert who also took part in such high profile cases as those of Susan Smith, convicted of killing her two children in a South Carolina lake; serial killer Jeffrey Dahmer; and "Unabomber" Ted Kaczynski.