Pope John Paul II's condition has stabilised following overnight medical treatment that included respiratory assistance, the Vatican has said.
The pontiff was last seen in public on Sunday
The 84-year-old pontiff was rushed to Rome's Gemelli hospital with breathing difficulties on Tuesday night after a bout of flu suddenly worsened.
The Vatican said in a statement later that the condition of the Pope's heart and lungs were "within normal limits".
A spokesman added that he would remain in hospital "for another few days".
Joaquin Navarro-Valls said the Polish-born pontiff had never lost consciousness. He still had a slight fever on Wednesday morning but had nonetheless participated in a Mass from his hospital bed.
"I think everyone has to be calm because there is no reason for alarm today," Mr Navarro-Valls told reporters outside the hospital on Wednesday morning.
Italy's Health Minister Girolamo Sirchia visited the Pope on Wednesday, saying afterwards: "He is improving, the doctors are optimistic."
The BBC's David Willey in Rome says the Vatican is at pains to stress that this is not a life-threatening illness.
But our correspondent says that because of the Pope's age and increasing frailty there is no question of setting a date when he will be able to leave hospital.
He will miss his weekly audience on Wednesday for the first time since September 2003, when he had an intestinal ailment.
Catholics around the world have attended special church services to pray for his recovery.
The Pope suffers from Parkinson's disease and painful joint conditions.
He was forced to cancel all engagements for several days after coming down with flu on Sunday.
The pontiff was taken by ambulance to the Gemelli hospital at about 2300 local time (2200 GMT) on Tuesday.
"The flu which has been affecting the Holy Father for three days was complicated with acute inflammation of the larynx and laryngo-spasm," Mr Navarro-Valls said in a statement at the time.
The Pope's latest health scare has caused concern around the world
Laryngo-spasm is a closure of the larynx that blocks the passage of air to the lungs - a condition where one cannot catch one's breath.
The Pope is being cared for in the same room at the hospital where he has been treated several times before.
Vatican sources say a contingency plan was made for the Pope to be taken to hospital if he fell victim to the current flu epidemic sweeping Italy. A set of rooms is always reserved for him on the 10th floor.
The appointment of Karol Wojtyla as the first Polish pope in 1978 was seen as a groundbreaking move for the Roman Catholic Church.
The first non-Italian pope in 455 years - and at 58, the youngest pope of the 20th Century - he was little known outside Vatican circles. Few experts tipped him as successor to Pope John Paul, who died after only 33 days in office.
John Paul II is now in the 27th year of his pontificate which makes him almost the longest-reigning pope in history.
The Pope - the leader of the world's estimated one billion Catholics - has visited more than 100 countries, and is estimated to have effectively circled the globe 27 times.