Kitzbuhel - Wladimir Klitschko's title defence against France's Jean-Marc Mormeck next weekend is in doubt.
The Ukrainian had to undergo an operation to have a kidney stone removed on Saturday.
"Only after the surgery will we be able to determine whether the fight can go ahead in a week's time," Klitschko's manager, Bernd Boente, said.
Klitschko, 35, left his training camp in Going near Kitzbuhel, Austria, on Friday because of renal colic pain and was flown to a hospital in Innsbruck.
The IBF, WBA and WBO heavyweight champion is due to face Mormeck, 39, in Düsseldorf next Saturday.
A decision as to whether the fight can go ahead is expected soon.
Klitschko's spokesperson had earlier said the fight would go ahead as planned, before it emerged the champion needed surgery.
Having beaten Britain's David Haye in July to add the WBA belt to his collection, Klitschko was scheduled for his 20th title fight.
His record is 56 wins, 49 knockouts and only three defeats.
The pre-fight press conference to be held on Monday has not been cancelled.