Pope Francis canceled a visit to a Rome hospital at the last minute on Friday because of what the Vatican called a "sudden indisposition".
The Vatican gave no immediate details about what was ailing the 77-year-old Argentine pontiff, who has canceled a number of engagements in the past few weeks due to minor health problems.
People had already gathered at Rome's Gemelli hospital to await the pope, who had been expected to visit patients and say mass, when the Vatican issued a statement saying Angelo Scola, Cardinal of Milan, would read a homily in his place.
Italian media said the pope had postponed the appointment because he was "very tired".
Earlier on Friday he had given his daily mass at Santa Marta in the Vatican, where he lives and held a series of audiences.
Users of Twitter, the social networking site where the pope's @Pontifex account has over 4 million followers, quickly wished him well, posting tweets including "Speedy recovery, Pope Francis" and "praying for the health of the pope".
The "popemobile" which carries the leader of the world's 1.2 billion Catholics short distances during official events, had arrived early at the Gemelli hospital and people had climbed on it to pose for photographs, Italian media reported.
The pontiff, who suffers from sciatica, or leg pain caused by a pinched nerve, canceled a scheduled walk in a procession through the streets of Rome last week, a decision the Vatican said was made to conserve his strength for coming engagements.
He canceled an engagement the week before due to a slight fever, but subsequently made a number of public appearances during which he seemed to be in good health.