Pope John Paul II has returned to the Rome hospital where he was treated earlier this month for flu-related breathing problems.
The Pope has been in frail health for several years
The Vatican said he suffered a relapse of his flu condition.
Reports say doctors are discussing whether to carry out surgery to help ease pressure on his inflamed windpipe.
The 84-year-old Pope spent 10 days in the Gemelli Hospital earlier this month suffering throat spasms brought on by the flu. He went home on 10 February.
The Pope seemed to have been recovering well, appearing twice at his Vatican study window to greet pilgrims.
THE POPE'S ILLNESS
1 Feb: Rushed to hospital suffering from "breathing difficulties brought on by flu"
4 Feb: Health improving and he is eating normally, Vatican says
6 Feb: Appears at window of hospital room and reads the final lines of the Angelus blessing in hoarse voice
9 Feb: Misses Ash Wednesday services at the Vatican for first time in 26 years
10 Feb: Returns to Vatican in motorcade, waving to crowds
13 Feb: Appears at Vatican for Sunday blessing for first time since leaving hospital
22 Feb: Launches new book in which he compares abortion with the Holocaust
24 Feb: Returns to Gemelli hospital after suffering a relapse of his flu condition
Vatican spokesman Joaquin Navarro-Valls said the Pope had suffered a return of flu symptoms on Wednesday afternoon and was admitted to hospital for further specialist treatment and checks on Thursday morning.
Italian media reports say doctors are considering performing a tracheotomy, where a small opening is cut into the neck and the windpipe so air can directly flow into the lungs.
An official medical bulletin on his condition is not expected until Friday.
Italian news agency Ansa said the Pope was conscious as he arrived at hospital. He is reported to have breathing difficulties and a fever.
The BBC's Robert Pigott says the Vatican has been keen to show the Pope's health in the best possible light to dispel speculation he may not be able to continue in the role.
He says the pontiff has looked frailer since his admission to hospital earlier this month.
On Wednesday the Pope gave a "virtual" general audience, addressing thousands of pilgrims via a television link from his study.
Although he sounded hoarse, the 25-minute address was his longest public appearance since leaving hospital.
But the pontiff, who has Parkinson's disease and arthritis, failed to attend a scheduled Vatican engagement on Thursday morning to approve the declaration of new saints.
It was the first time he had missed the ceremony, known as a consistory, in his 26 years as Pope.
During his previous spell in hospital, he had also missed Ash Wednesday services at the Vatican for the first time while pontiff.
At times in the past three years he has had difficulty completing speeches and tires very quickly during public appearances.
The appearance of the pontiff, with his bent frame and trembling hands, sometimes shocks pilgrims who attend audiences at the Vatican.