Pope Leo XIV had a cancerous growth removed from his right lung in a 90-minute operation on Tuesday and "woke up normally" after the procedure, the Vatican said.
A Vatican source said further tests were being carried out to determine if the cancer had spread.
"To ensure the procedure was radical, the lower lobe of the right lung was removed," the Vatican said in a statement.
The operation "confirmed the diagnosis of a pulmonary adenocarcinoma, as suspected from the pre-operative tests", it said.
The pope was in the operating room for a total of three hours and the operation lasted half that time.
"The surgery was successful and there were no complications," the Vatican said.
A Vatican source said he requested dinner after the procedure.
Leo had entered the hospital smiling and waving earlier on Tuesday hours before the surgery.
The Vatican has said it expects the pope to stay in his room on the 10th floor of Rome's Gemelli hospital to recover for several days.
His audiences have been cancelled until October 20 and the operation raises questions about whether the pope will go on a lengthy visit to Latin America next month.
"I will pray a lot"
The surgery marks the 71-year-old Leo's first hospitalisation since he became head of the Catholic Church on May 8, 2025.
In a surprise announcement following Angelus prayers on Sunday, Leo told crowds in St Peter's Square that doctors had discovered a nodule "at a very early stage" during medical checks.
"The doctors believe it would be right to intervene," he said.
The Vatican has historically been very secretive about the health of popes, but began to be more transparent during the illness of Leo's predecessor Francis, an Argentine who freely discussed his ailments.
Ahead of the operation, people outside the hospital expressed concern over the pope's condition.
Giorgia Fei, a 57-year-old Italian, said she had been worried.
"However, the Holy Father is in very good hands here at the hospital because we have some outstanding doctors," she said.
"I will pray a lot for him."
Giuseppe Purpura, 72, said he too was concerned.
"He's still quite young and I really wasn't expecting this. I thought he was in better health," he said, noting that "thankfully it was caught in time."
The Vicar General for the Diocese of Rome, Cardinal Baldassare Reina, led a rosary prayer for the pope inside the hospital chapel which was broadcast by the Vatican. Dozens of people attended and Reina asked all the city's parishes to join in.
The Vatican secretary of state, Cardinal Pietro Parolin, told ANSA news agency that Leo had been "very calm" about his condition when they met on Monday.
The Vatican told AFP the nodule was found during routine health checks, and there were no symptoms that led to checking the pope's lungs.
Chicago-born Leo, the first US pope, is sporty and believed to generally be in good health.
He is scheduled to travel to South America from November 6-17 to visit Uruguay, Argentina and Peru.
It was not immediately clear what would happen if the pope was forced into a longer recovery.
Under Church law, a figure known as the Cardinal Camerlengo -- or chamberlain -- takes over administrative duties if the pope dies, but pontiffs do not have a deputy to replace them if they become incapacitated.
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