Pope Francis has a respiratory infection and will need to spend "a few days" in hospital for treatment, the Vatican said in a statement on Wednesday.
The Vatican said the 86-year-old pope had complained of breathing difficulties in recent days. It added that tests showed he did not have COVID-19.
Pope Francis has gone to Rome's Gemelli hospital for a previously scheduled check-up, his spokesman Matteo Bruni said in a brief statement on Wednesday, giving no further information.
A Vatican source said the pope's diary was being cleared to allow for medical tests to continue.
The website of Corriere della Sera said the pope was taken to hospital by ambulance after he had suffered "heart problems."
Francis had appeared in good health
The 86-year-old pope earlier in the day attended the weekly general audience at the Vatican and appeared in good health.
Pope Francis suffers from diverticulitis, a condition that can infect or inflame the colon, and had an operation at the Gemelli hospital in 2021 to remove part of his colon.
Earlier this year, he said the condition had returned and that it was causing him to put on weight, but that he was not overly concerned. He did not elaborate.
He also has a problem with his knee and alternates between using a cane and a wheelchair in his public appearances.
Francis told Reuters in an interview last year that he preferred not to have surgery on his knee because he did not want a repeat of long-term negative side effects from anesthesia that he suffered after the 2021 operation.
Last July, returning from a trip to Canada, Francis acknowledged that his advancing age and his difficulty walking might have ushered in a new, slower phase of his papacy.
But since then, he has visited Kazakhstan and Bahrain and made a grueling trip last month to Democratic Republic of Congo and South Sudan.
He has also said this year that he has no plans to resign anytime soon and that if he eventually did it would be for serious heath reasons, such as if he were gravely incapacitated.
Asked by Italian Swiss television RSI in an interview broadcast on March 12 what condition would lead him to quit, he said "A tiredness that doesn't let you see things clearly. A lack of clarity, of knowing how to evaluate situations."