His daughter, Maria Gaidar, told RFE/RL he began vomiting and fainted during a presentation at an economic conference in Dublin on November 24.
"He became sick during his speech," she said. "As he was leaving [the room], he fainted and remained unconscious for three hours. He was taken to the emergency room of a Dublin hospital. There was a real threat to his life."
Doctors say Gaidar's condition has stabilized. So far, however, they have been unable to determine the cause of his illness.
Maria Gaidar said her father received a clean bill of health during a physical exam just one month ago, and that medical officials have ruled out the possibility that Gaidar suffered a heart attack, stroke, or an onset of diabetes.
Gaidar's sudden illness, coming one day after the mysterious death of the former intelligence officer and longtime Kremlin critic Aleksandr Litvinenko, has sparked suggestions that Gaidar, too, was targeted for assassination.
Anatoly Chubais, a longtime associate of Gaidar and the head of Russia's national electricity monopoly, says he suspects a connection between Gaidar's illness, Litvinenko's poisoning, and the murder in October of investigative reporter Anna Politkovskaya.
He said people seeking what he called an "unconstitutional and forceful change of power in Russia" could have been behind the three incidents.
Aleksandr Khinshtein, a Duma deputy with pro-Kremlin Unified Russia, dismissed such accusations.
Khinshtein told journalists the recent reports about Litvinenko and Gaidar are part of a "complex plan, developed in the West, to discredit the Russian leadership and special services, and, specifically, President [Vladimir] Putin."
The leader of Russia's Union of Rightist Forces (SPS), Nikita Belykh, told RFE/RL he believed there was reason to suspect foul play but that he did not think there was a link between Litvinenko's death and Gaidar's illness.
"Of course, I can assume that if this was indeed a poisoning attempt, it might have political motives. But it is too early to talk about it and I would not link Litvinenko's death and Yegor Timurovich [Gaidar's] poisoning," Belykh said.
He added that he had never heard Gaidar mention any threats made against him, although he added that Gaidar had "enemies inside and outside Russia."
Maria Gaidar said her father had not suggested to her he had been poisoned, and that she was reluctant to "rush to conclusions."
"As far as I understand, he didn't have such a version. I didn't hear him say that," she told RFE/RL. "Such conclusions can be drawn from conversation with witnesses, with people who saw him in Dublin, so such allegations are indeed being made. But I didn't hear him make any such allegations."
Gaidar's press secretary, Valery Natarov, told Interfax that the possibility Gaidar had been poisoned cannot be ruled out, but added, "what is ruled out is poisoning with the use of radioactive isotopes."
Litvinenko, who died November 23 after a severe, three-week illness, was said by medical officials to have died from exposure to the radioactive isotope polonium-210.
Mourners in Moscow mark the 40th day after the killing of investigative journalist Anna Politkovsksya on November 15 (TASS)
DANGEROUS DISSENT. A surprising number of vocal critics of the policies of Russian President Vladimir Putin have been killed or have died mysteriously in recent years. Although the Russian government has denied any involvement in any of the cases, some Kremlin watchers have begun speaking of a clandestine campaign to eliminate dissent.
April 17, 2003: Sergei Yushenkov, veteran liberal politician, Duma member, and leader of a staunchly anti-Kremlin party, is shot dead in Moscow.
July 3, 2003:
, liberal lawmaker and investigative journalist, dies of a mysterious allergic reaction. Many believe it was a case of deliberate poisoning, but the incident was never investigated as a murder.
February 13, 2004:
Former acting Chechen President Zelimkhan Yandarbiyev
is killed in an explosion in Doha, Qatar. Two Russian security-service agents are later convicted of carrying out the killing.
September 2, 2004: Investigative journalist Anna Politkovskaya falls ill mysteriously on a plane bound for the North Caucasus. Politkovskaya was heading to Beslan, North Ossetia, in a bid to negotiate the release of schoolchildren being held hostage there by Chechen militants.
Former Ukrainian Prime Minister Viktor Yushchenko
, running for president as a pro-Western candidate against Prime Minister Viktor Yanukovych, is poisoned. He recovers and goes on to win the presidency, although his poisoning remains a mystery.
October 7, 2006: Investigative journalist and vocal critic of Russian policies in the North Caucasus Anna Politkovskaya is gunned down in Moscow.
November 23, 2006: Former Federal Security Service agent Aleksandr Litvinenko, a vocal critic of Russia's secret services, dies of a mysterious poisoning in London.
An annotated timeline
of high-profile killings in Russia since the collapse of the Soviet Union.
For news and analysis on Russia by e-mail, subscribe to "RFE/RL Russia Report."