Kathleen Savio's body was found in a bathtub in 2004. Originally ruled an accident, the case was reopened after Peterson's fourth wife, Stacy Peterson, went missing in 2007.
At issue- testimony from Jeff Pacther, who says Peterson asked him to kill Savio just months before her murder. The prosecution had left that information out of a pre-trial document, apparently an oversight, and the defense had argued it should stay that way.
Previously, Judge Burmila had not allowed the hit-man allegation to be raised in front of jurors, but Tuesday's decision allows prosecutors to introduce the evidence for a narrow purpose.
"The issue is not whether he wanted to hire a hit man," Judge Burmila said. "The issue is: Did the defendant intend to kill his wife? ... This evidence goes to that matter."
"It's excellent for the state, I think it should be. It really ties into the motive that Drew was looking for someone to kill Kathleen," Pam Bosco, Stacy Peterson family spokesperson said.
Peterson's attorneys dismissed the importance of the state's big legal victory.
"If that is what they are relying on in this case, it's weaker than I thought," Peterson defense attorney Joel Brodsky said.
At a 2010 hearing, Pachter testified that - as the two rode in Peterson's squad car - Peterson asked if he could find someone to "have his third wife taken care of." Pachter said he took that to mean he wanted her murdered, though Peterson didn't use that word.
Pachter said he didn't take Peterson's offer seriously, saying he simply responded, "OK," but did nothing about it.
Peterson explained to him that he asked Pachter partly because Pachter worked in a dangerous area, and Pachter understood that to mean Peterson believed he would be able to find a drug dealer or gang member to carry out the job.
At the hearing two years ago, Pachter said he hadn't heard about Savio's death in March 2004 until he telephoned Peterson the following July about another matter. He said Peterson said during the call, "By the way, the favor that I asked you, I don't need it anymore."
Pathologist: Savio's wound wouldn't cause her to pass out, drown
Because it is a circumstantial case, prosecutors are relying on everything, especially testimony from their forensic pathology experts. On Tuesday, Dr. Mary Case countered the defense theory that Savio died from hitting her head after she slipped and fell in the bathtub.
Showing close-up photos of Savio's brain, Dr. Case told the jury Savio's head injury is not the type that would cause her to lose consciousness and drown.
Case also said the injury could not be caused by the smooth surface of a bathtub and that in her opinion, Savio's death was a homicide, meaning, "some other person did it." She ruled out accidental or suicide as cause of death.
"She is obviously tailoring her opinion for the result she wants to get. That's she gets paid," Brodsky said.
Peterson has pleaded not guilty in the 2004 death of his third wife, Kathleen Savio. He is also a suspect in the disappearance of his fourth wife, Stacy Peterson, but no charges have been filed. He maintains his innocence in both cases.
Peterson jurors wear color-coordinated outfits
There's apparently a daily color-coordination memo circulating the jury room at Drew Peterson's murder trial.
Jurors hearing the case outside Chicago have entered court each day for at least a week wearing the same colored clothing.
Tuesday, virtually all wore some shade of green. Before starting a three-day break Friday, they wore black. On Thursday, it was blue and Wednesday it was red.
The unusual display has left courthouse observers scratching their heads. But there's no way to tell what the jurors are thinking. They're barred from communicating with reporters or other trial participants.
Defense attorney Joel Brodsky says he noticed the color coordination but it doesn't bother him.
The Associated Press contributed to this report. All rights reserved.