The Kremlin said that the doctors who examined Mr Yeltsin had ordered the 65-year-old president back to the Central Clinical Hospital in Moscow where he recuperated after his quintuple by-pass on 5 November last year.
The president, who had been looking pale and tired, had already cancelled all appointments since Monday because of what the Kremlin said was a heavy cold. Doctors said the cold was not related to the operation and did not threaten his heart.
"Doctors have stated that the first signs of pneumonia developing have appeared," a Kremlin statement said. "In connection with this ... a decision has been taken to take him to the Central Clinical Hospital for several days."
His spokesman at first said he had a cold, but later upgraded it to flu - a bug which is sweeping Moscow.
The pioneering American heart surgeon Michael DeBakey, who advised the Russian doctors who operated on Mr Yeltsin, said he expected the president to make a quick recovery.
But a return to his sickbed only a fortnight after returning to work at the Kremlin is a serious blow to Mr Yeltsin, and opens the way to feuding among his would-be successors such as bedevilled the period running up to his by-pass.Reuse content