Doctors at Vienna's elite Rudolfiner clinic said tests run over the weekend proved beyond a doubt that it was dioxin poisoning that caused a mystery illness in September that left Yushchenko disfigured and in pain.
"I don't want this factor to influence the election in some way — either as a plus or a minus," Yuschenko said in Russian as he left the clinic and headed back to Kiev. "This question will require a great deal of time and serious investigation. Let us do it after the election — today is not the moment."
Following the discovery of the dioxin, Ukraine's prosecutor general's office said it had reopened the criminal investigation that it closed in November for lack of evidence of poisoning.
Suspicion may fall on the Kremlin, reports CBS News Correspondent Tom Fenton, who notes Moscow fears Yushchenko will take Ukraine out of Russia's orbit by joining NATO and the European Union.
"But," adds Fenton, "it looks like this will now propel him into office, unless something else happens to him before the election."
A Russian health ministry official questioned the doctors' conclusions, saying in a radio interview that dioxin does not work immediately.
While high concentrations of dioxin remain in his blood, doctors said Yushchenko's organs have not been damaged and he is fit for the campaign trail.
Yushchenko no longer needs the catheter in his spine that had administered medication to treat intense back pain, but still is taking painkillers, Zimpfer said.
The 50-year-old opposition leader thanked the medical staff as he checked out of the clinic. Doctors said that if the dose of dioxin had been greater, it could have been fatal.
"They've spent many days and nights with me and I am very happy to be alive in this world today," Yushchenko said, with his American-born wife, Kateryna Chumachenko, translating. "I thank these people for this."
Lawmakers from Yushchenko's party said the clinic's findings confirmed that his opponents wanted to assassinate or disable him rather than take the risk he would defeat Prime Minister Viktor Yanukovych in the presidential election.
Kremlin-backed Yanukovych won the initial presidential runoff, but the Supreme Court voided the vote on fraud allegations.
After doctors confirmed Saturday that Yushchenko was a victim of poisoning, Yanukovych campaigners rejected suggestions that the prime minister could have been involved.
There is "no logic in such an accusation," said Taras Chornovyl, Yanukovych's campaign manager.
At a brief news conference before he was discharged from the Vienna clinic, Yushchenko praised the thousands in Ukraine who staged street protests against the outcome of the Nov. 21 runoff.
"We haven't seen anything like that for the past 100 years," he said. "I think it would be appropriate to compare this to the fall of the Soviet Union or the fall of the Berlin Wall."
Yushchenko fell ill Sept. 5 and has been treated at the Vienna clinic twice before, but it was tests performed since he checked in Friday night that provided conclusive evidence of the poisoning, Zimpfer said.
A lab in Amsterdam, using a newly developed test, found his blood contained more than 1,000 times the normal amount of dioxin, Zimpfer said.
Tests showed the toxin was taken orally, and was likely slipped into something that Yushchenko ate or drank, Zimpfer said, suggesting that whoever was responsible may have thought it untraceable.
"Until recently, there has been no (blood) testing available" for dioxin, Zimpfer said. "This may be one of the reasons that this kind of poisoning, if it was a criminal act, was chosen."
The tests definitively confirmed suspicions that the doctors had developed over the course of Yushchenko's treatment, Zimpfer said.
"This is the first case internationally where the intake has been oral, usually it's inhaled, it's very different," he said.
Dioxin is a byproduct of industrial processes such as waste incineration and chemical and pesticide manufacturing.
The massive quantities of it found in Yushchenko's system caused chloracne, a type of adult acne caused by exposure to toxic chemicals. The condition is treatable, but can take two to three years to heal.
Given the sensitivity of the case, Zimpfer said his clinic wanted to be absolutely sure before making any sort of announcement.
"We are not dealing with simple pimples, we are dealing with a poisoning and the suspicion of third party involvement, so potentially a criminal case," he said.
Zimpfer said Yushchenko's treatment will now be "very difficult and long."
Among other things, Dioxin is known to cause cancer, and Dr. Nikolai Korpan, the physician who has been treating Yushchenko, said it was too early to tell what other problems might develop.
For now, he said, "we can confirm that his health is very good at this moment and he can do his job," Korpan said.