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 statement said the 86-year-old pontiff had complained of breathing difficulties in recent days. It added that tests showed he did not have COVID-19.
"Pope Francis is touched by the many messages received and expresses his gratitude for the closeness and prayer," the Vatican said.
The statement came hours after the pope was unexpectedly taken to hospital for tests.
The Vatican initially said the check-up had been scheduled, but Italian media questioned that, saying a television interview with the pope set for Wednesday afternoon had been cancelled at the last moment.
Earlier in the day, the pope attended his weekly general audience at the Vatican and had 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.
He said in January that 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 anaesthesia 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 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".
The pope was due to preside over a Palm Sunday service on April 2, the start of what is a hectic week of Easter ceremonies.
