Ukrainian opposition leader and presidential candidate Viktor Yushchenko (search) arrived at a private Vienna hospital Friday for tests to determine the cause of mysterious illness that has disfigured his face.

Arriving at the Rudolfinerhaus clinic (search), Yushchenko said he would stay until Monday, unless the tests were finished before then.

Yushchenko, 50, first fell ill in September and was rushed to the Vienna hospital. He resumed campaigning later in the month but with a pockmarked and badly disfigured face.

He has accused Ukrainian authorities of trying to poison him ahead of Ukraine's presidential vote — an allegation they have denied.

His chief of staff, Oleh Rybachuk, told PBS' "NewsHour with Jim Lehrer" that Yushchenko has fully recovered from his unidentified ailment, "but he needs a certain rest and he needs to take care of this effect on his face, which they call residual."

Rybachuk also said Yushchenko was fortunate to receive earlier treatment in Vienna.

"He was very lucky that he was brought to Vienna because doctors said if he would stay another 24 hours in Ukraine, it could be a 'final solution,"' he said in comments broadcast Friday.

Earlier this week, doctors said they were testing several theories as to what caused Yushchenko's ailment.

Dr. Michael Zimpfer, director of Rudolfinerhaus had said they were "investigating the hypothesis of poisoning. However, we have not found any indication that a chemical or biological substance has been employed. Also, we are following new threads, and (we have) included other labs to do more specific testing."

Yushchenko suffered from a series of symptoms, including back pain, acute pancreatitis and nerve paralysis on the left side of his face.

Doctors have only "a descriptive diagnosis" but no proof of what led to the ailments, Zimpfer said, adding they could have had internal causes or been sparked by a poison.

"It might also have been a combination of poisons. Everything is in the air," he said.

Ukraine's (search) Supreme Court voided the outcome of the Nov. 21 presidential runoff election, which Yushchenko lost to Prime Minister Viktor Yanukovych, following allegations of massive vote fraud. A rerun of the ballot is slated for Dec. 26.

Yushchenko told reporters in Vienna that he was confident of winning the election and that, in reality, he had already done so.