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
Gemelli: The Pope's hospital
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.
Ceremony missed
Although he sounded hoarse, the 25-minute address was his longest public appearance since leaving hospital.
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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.
Vatican officials say the pope has been readmited to hospital
Pope John Paul II has been re-admitted to Rome's Gemelli Hospital with a relapse of influenza which saw him hospitalised earlier this month.  

He was wheeled into Gemelli Hospital on a partially elevated stretcher, and was waving to people as he entered the clinic.

Later, Vatican spokesman Joaquin Navarro Valls announced that the pope had undergone a tracheotomy to assist his breathing.

The Pope's re-admittance again raises questions about how long his papacy will last.  One of his close aides refused to rule out the possibility of the Pope retiring saying it was up to his "conscience".  The Pope has recently indicated that he has no intention of retiring by saying that the call "to guard his flock" was "particularly alive in him".  

The operation lasted around thirty minutes, and Navarro Valls said to media that it was a success.