President Boris Yeltsin, in another health setback, was hospitalized today for what aides described as pneumonia, only days after he came down with a virus and bronchitis.

The 68-year-old president was taken to the Central Clinical Hospital from his out-of-town residence known as Gorky-9. Russian officials were tight-lipped about his condition tonight.

The Interfax news agency reported that doctors suspected pneumonia during Yeltsin's regular morning checkup and suggested he be hospitalized, according to press secretary Dmitri Yakushkin. He also said Yeltsin had working meetings in the morning, including one with his chief of staff Alexander Voloshin, and was "working with documents."

In the past, this phrase has often been used to mask more serious illnesses, which the Kremlin has disclosed only later. Yakushkin had told journalists this morning that Yeltsin was being treated for a "cold" at his residence and that his condition was "satisfactory."

Yeltsin, who underwent coronary artery bypass surgery in 1996, has a long history of health problems that have become more acute since his reelection that year. He has suffered from an ulcer and respiratory illnesses, often during the winter months.

Some unofficial sources have said Yeltsin also suffers from circulatory difficulties, which cause him periodically to zone out, but the Kremlin has never acknowledged this. However, Yeltsin often disappears from public view for weeks at a time.

Just last week, a planned ceremony for signing a treaty to bring Russia and Belarus closer was postponed because Yeltsin fell ill. Russian officials said it would be revived soon, but today aides said Yeltsin would remain in the hospital for at least a week.

If Yeltsin were to die or be incapacitated, Prime Minister Vladimir Putin would become acting president for three months until elections are held. However, the 1993 Russian Constitution did not specify what constitutes incapacitation or how it is to be determined.

Yeltsin appeared to be vigorous during his recent visit to the European security summit in Istanbul, but soon thereafter fell ill with what was diagnosed as bronchitis and a virus. Yeltsin was reported by aides to be hoarse, and Russian television said he was drinking warm milk and honey over the weekend. Russian politicians have been extremely nervous about signs that Yeltsin is ill because of the uncertain course of the war in Chechnya and the parliamentary elections Dec. 19.

CAPTION: Boris Yeltsin, 68, suffers a setback in addition to a virus and bronchitis.