Pope Leo is to undergo surgery this week to remove a nodule on his lung.
He made the announcement during the Angelus, his weekly Sunday blessing in Rome's St Peter's Square.
Leo told gathered pilgrims: "Recently, during some medical examinations, doctors discovered a well-defined lung nodule, detected at a very early stage and clearly localised.
"The doctors consider it appropriate to intervene, and so, following their advice, I have decided to undergo surgery to have it removed.
"The procedure has already been scheduled for the coming days. I thank you and entrust myself to your prayers."
Matteo Bruni, who is director of the Holy See's press office told journalists that the operation "is scheduled for the first days of this week and the estimated post-operative recovery time will be about one week."
The Pope, who's 71, has no previously known health problems and is physically active, enjoying tennis and swimming.