In a statement posted on its Web site, Ukraine's State Security Service, or SBU, said that "it has no relation with the worsening" of Yushchenko's health.
Last week in an interview with The Associated Press, Yushchenko said he was probably poisoned at a Sept. 5 dinner with the head of the Ukrainian security service Ihor Smeshko and his deputy, Volodymyr Satsyuk, who later denied any involvement in the poisoning.
"The Ukrainian Security Service did not obtain a single official document that could provide ... a basis for the establishment of the time or the place or the fact of the candidate's poisoning," the statement said.
The poisoning dramatically disfigured Yushchenko's face but doctors have said he has recovered enough to campaign.
Ukrainian prosecutors and a special parliamentary committee are investigating and the state security service has expressed willingness to participate in the probe.
On Wednesday, Yushchenko told thousands of his orange-clad supporters massed at Kiev's Independence square for a giant rally that his victory is near.
"The doors have been opened. The only thing left for us is to step over the threshold," Yushchenko said during the open-air speech called to mark one month since the "orange revolution" protests.
He warned of a plot to disrupt the revote, but did not say who was behind it.
"The vote on Dec. 26 will not be an easy political walk," he said. "There are some forces preparing to disrupt and they are preparing brigades, groups who are readying to come to Kiev."
"We will come on this square, this stage, after the vote on Dec. 26 and will stay until our victory is celebrated," he said.
Fears of violence have been high ahead of Sunday's runoff, with rumors swirling that armed pro-Yanukovych supporters are poised to head to Kiev after the vote. Yanukovych's campaign staff has repeatedly denied the allegations.
The heated campaign has divided Ukraine between the pro-Yanukovych industrial, Russian-speaking east, and the west and center where Yushchenko draws his support. Some eastern regions had raised the possibility of pursuing autonomy if Yushchenko wins.
Yushchenko's call for supporters to gather after the vote echoed a similar appeal he made after the Nov. 21 runoff that Yanukovych won, until the Supreme Court annulled the vote, citing massive fraud.
For nearly three weeks, Independence Square was the scene of massive protests that paralyzed the government in this ex-Soviet republic. Protesters set up a sprawling tent camp on the tree-lined main street, bringing central Kiev to a halt.
Yanukovych has openly warned that even if Yushchenko wins, he will never be considered president of all of Ukraine.
Yushchenko, relying on the protests and the outcry over the fraudulent vote, has found himself leading over a weakened and increasingly isolated opponent. Yanukovych has been abandoned by outgoing President Leonid Kuchma and appears to be losing the support of the Kremlin.
Also Wednesday, Yushchenko pledged to make Russia his first official destination if he is elected, a statement that came after Russian President Vladimir Putin promised to work with whomever wins the presidency.
But Yushchenko also insisted that attempts to make Russian the country's second official language had become a political issue and required extensive discussion.
Almost 70 percent of the population uses Russian on a daily basis, but Ukrainian predominates in the more nationalistic west. Yanukovych supporters fear a Yushchenko presidency would lead to discrimination against Russian speakers, as has happened in other former Soviet republics.
Yushchenko has repeatedly said he would defend Ukrainians' right to speak whatever language they want.