Barbaro was being treated aggressively for his discomfort and is in stable condition, according to a statement released Wednesday morning by the University of Pennsylvania's New Bolton Center.

The tissue was removed Tuesday night.

"Things were marching along pretty smoothly until this," Barbaro's co-owner Gretchen Jackson said. "We've been there before with him. He's a horse that wants to live."

The setback comes one week after a new cast was placed on Barbaro's laminitis-stricken left hind foot to help realign a bone.

The cast change could have caused some inflammation, said Dr. Kathleen Anderson, Barbaro's attending vet when the horse was racing and stabled in trainer Michael Matz's barn at the Fair Hill Training Center in Elkton, Md.

Anderson said Barbaro has proved he was strong enough to overcome his latest medical obstacle.

"We all know most horses don't get this far," she said. "The bottom line with Barbaro is the fractured leg is the one that would have been the end of most horses. He won't be getting to the big green field any time soon, but I don't think this is insurmountable."

It was the first dose of bad news after months of progress that included owners Gretchen and Roy Jackson and New Bolton's chief surgeon Dean Richardson talking about releasing Barbaro from the hospital as soon as the end of the month.

"It's sad that's he's had a setback because he was marching along toward living outside the hospital," Jackson said. "The only thing we care about is that he's not in pain."

Barbaro had become uncomfortable on his left hind foot and a cast was removed after some new separation on the inside portion of his hoof was found.

Barbaro shattered his right hind leg in the Preakness on May 20. In mid-July severe laminitis, a potentially fatal disease caused by uneven weight distribution in the limbs, resulted in 80 percent of Barbaro's left hind hoof being removed.

"I was there yesterday and it was obvious he was not comfortable in that foot," Jackson said. "The easiest and best way to work on Barbaro is when he's laying down. They had to wait until he was laying down and when they removed the cast, they discovered some reason for him feeling pain."

Just over a week ago, Richardson said Barbaro's right hind was getting stronger and should eventually be healthy enough to allow the colt to live a comfortable, happy life.