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.
HAVE YOUR SAY I hope the Holy Father will soon be well again
Teresa, Cospicua, Malta
Send us your comments
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.
Plaza de San Pedro, Ciudad del Vaticano

El papa Juan Pablo II ha sido recluido nuevamente en la Clínica Gemelli de Roma, debido a una recaída por la gripe que sufre desde hace varias semanas y que lo había internado a comienzos de este mes en el mismo centro asistencial. El pontífice fue llevado en silla de ruedas al interior del hospital, mientras saludaba la multitud.

Horas más tarde, el vocero del Vaticano, Joaquín Navarro-Valls anunció que el pontífice había sido sometido a una traqueostomía, con el fin de mejorar su respiración. La operación habría durado 30 minutos y habría sido exitosa, según Navarro. En la mañana del viernes, Juan Pablo II habría "desayunado en abundancia" luego de haber pasado una noche tranquila en el hospital. No obstante, el jerarca católico no podrá hablar durante algunos días, debido a la operación. Por eso, se ha armado de un cuaderno en el cual escribirá sus necesidades personales y sus decisiones acerca de la Iglesia.

Esta nueva hospitalización abre la discusión acerca de la duración del pontificado de Karol Wojtyla. No obstante, fuentes cercanas al pontífice aseguran que no piensa dimitir. De hecho, durante esta semana estuvo en constante actividad, incluyendo una videoconferencia realizada el miércoles.