Richard Albert Ricci underwent six hours of emergency surgery to remove a blood clot from his brain.
"He did have pressure on his brain, and it did cause some of his brain to not function the way it should," said neurosurgeon Dr. Elaine Skalabrin, adding that the next 48 hours should show whether he responds to the surgery.
Doctors don't know the cause of the apparent cerebral hemorrhage.
A spokeswoman for Ricci's wife, Angela - who has defended him from day one - notes that he has a history of high blood pressure and has been under tremendous stress as a result of the public scrutiny in the Smart case.
"This is the most bizarre thing that I ever could have imagined," Elizabeth's father, Ed Smart, said during a news briefing Wednesday.
Salt Lake City Police Chief Rick Dinse said his first reaction upon hearing of Ricci's condition was to swear.
"Clearly, if he does not survive, that would be a big impact on the case," Dinse said at the briefing.
Ricci has maintained his innocence and has not been charged in the disappearance of 14-year-old Elizabeth, who was taken from her bedroom by a gunman on June 5.
Ricci, a former handyman for the Smart family, was in prison Tuesday on a parole violation on theft and burglary charges when he called guards to his cell and said he was having trouble breathing, state Corrections spokesman Jack Ford said. As they talked to him, he passed out.
They began resuscitation, and he was flown to University Hospital in Salt Lake City.
Salt Lake County Sheriff's Lt. Robby Russo said Ricci, 48, might have been brain-dead by the time he arrived at the hospital. "Now, it's too early to tell if he will have brain damage if he survives," Russo said.
No traces of drugs were found in Ricci's cell, and Ford said Ricci didn't leave a note to indicate suicide.
Afterward, Smart said his first thought was that Ricci had been poisoned, even though he was being held in a maximum-security cell.
Smart said he hoped that if Ricci survived, his near-death experience would "soften his heart."
Police hoped survival would make Ricci more cooperative. "He's an individual who generally speaking never volunteered a lot of information," Dinse said. "He has told us things we don't believe are true."
Ricci had been in court earlier Tuesday, when his attorneys asked for more time to review burglary and theft charges that could result in a life sentence for Ricci, if he is convicted. Third District Judge Paul Maughan granted the request and a hearing was scheduled for Sept. 17.
The theft charges include allegations that he took items from the Smarts' home.