"Following a sudden indisposition, the Holy Father will not be going this afternoon to the Gemelli hospital," it said in a statement, adding that a cardinal would take his place.
It is not the first time the hardworking 77-year-old pontiff has cancelled programmed events at the last minute.
"I cannot say whether the pontiff is suffering from tiredness or indigestion," Vatican spokesman Federico Lombardi told AFP.
"With so many undertakings, it's clear there's a need for a break every now and then," he said, describing "the intense pace" of the Argentine's schedule.