The Vatican said Wednesday that Pope Francis, 86, was in hospital in Rome for previously scheduled health checks, without giving further details.
"The Holy Father has been at Gemelli (Hospital) since this afternoon for some previously scheduled checks," wrote Vatican spokesman Matteo Bruni in a one-sentence statement.
The pontiff, who earlier this month marked 10 years as head of the Catholic Church, had earlier appeared in good spirits at his weekly audience at the Vatican, smiling as he greeted the faithful from his "popemobile".
However, he was seen grimacing as he was helped getting into the vehicle.
Pope Francis suffers from chronic knee pain that has forced him to rely on a wheelchair in recent months.
The Gemelli was the same hospital where the pope underwent an operation on his colon in July 2021 after suffering from a type of diverticulitis, an inflammation of pockets that develop in the lining of the intestine.
He remained in hospital for 10 days. A year later, in July 2022, he admitted he was still feeling the effects of six hours spent under anaesthetic during the surgery.
Pope Francis had to cancel or curtail activities several times last year because of the pain in his knee and in a July 2022 interview acknowledged that he needed to slow down.
Francis' health has been the frequent subject of speculation, particularly the question of whether he will follow the example set by his predecessor and retire if he cannot continue.
Benedict XVI, an eminent German theologian, shocked the world in 2013 by becoming the first pope since the Middle Ages to resign.
The two "men in white" co-existed within the walls of the tiny Vatican state for almost a decade, before Benedict died on December 31.
Francis has said he would follow Benedict in stepping down if his health made him unable to do his job.
However, he told an interviewer in February that papal resignations should not become "a normal thing", adding that for the moment it was not on his agenda.
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Despite his advancing age and health problems, Francis continues to travel widely.
Huge crowds greeted him on a visit earlier this year to South Sudan and the Democratic Republic of Congo, a testament to his ongoing popularity.
In the past decade, he has sought to forge an image of a more open, compassionate Church, although has faced internal opposition, particularly from conservatives.
Next month, Pope Francis is due to visit Hungary and meet Prime Minister Viktor Orban.
The Argentine almost died when he was 21 after developing pleurisy — an inflammation of the tissues that surround the lung — according to biographer Austen Ivereigh.
He had part of one of his lungs removed in October 1957.
He has also talked about the surgical removal of cysts from the top lobe of his right lung.
He insisted he had made "a complete recovery… and never felt any limitation since then".