Veteran US Republican Senator John McCain has been diagnosed with brain cancer and is reviewing treatment options, according to his office.
The options may include chemotherapy and radiation, his doctors said. The 80-year-old politician is in "good spirits" recovering at home.
He thanked those who had wished him well and said he would be back soon.
The tumour was discovered during a surgery to remove a blood clot from above his left eye last week.
A Vietnam veteran, Mr McCain spent more than five years as a prisoner of war.
The six-term senator and 2008 Republican presidential candidate underwent surgery at a clinic in Phoenix, in the state of Arizona, last Friday.
Tissue analysis revealed that a primary brain tumour known as glioblastoma was associated with the clot, a statement from the Mayo Clinic said.
"The senator's doctors say he is recovering from his surgery 'amazingly well' and his underlying health is excellent," it added.
"Treatment options may include a combination of chemotherapy and radiation."
Senior Republicans and Democrats wished him a speedy recovery, prompting Mr McCain to tweet his thanks, and a warning:
"I greatly appreciate the outpouring of support - unfortunately for my sparring partners in Congress, I'll be back soon, so stand-by!"
Analysis: Warrior faces a new battle
By James Cook, BBC North America correspondent
John McCain is known in Washington as a tough, independent-minded senator - a warrior who is now facing another battle against cancer.
He earned his reputation the hard way, being shot down as a US Navy pilot over Vietnam where he was held as a prisoner of war for more than five years, including two in solitary confinement.
Repeatedly beaten and tortured, Mr McCain was never again able to raise his arms above his head.
During the most recent presidential election campaign, Donald Trump belittled the senator as "not a war hero" saying "I like people who weren't captured".
Many Americans were horrified.
Mr McCain may have annoyed many Republicans by arguing for reforms to campaign finance and immigration laws.
He may have irritated opponents of America's many wars with his forceful arguments in favour of the projection of US military might.
But this country reveres its veterans. The attacks on John McCain's personal sacrifice were roundly condemned then - and millions of Americans will be praying for his recovery now.
Glioblastoma is a particularly aggressive brain tumour, and increases in frequency with age, affecting more men than women.
Mr McCain, who is the chairman of the Senate Committee on Armed Services, was in "good spirits as he continues to recover at home with his family", his office said.
His family reacted with "shock" to the news, his 32-year-old daughter Meghan said.
"It won't surprise you to learn that in all of this, the one of us who is most confident and calm is my father," she said on Twitter.
"So he is meeting this challenge as he has every other. Cancer may afflict him in many ways: but it will not make him surrender. Nothing ever has."
President Donald Trump said Mr McCain had "always been a fighter" and, in a statement, said: "Get well soon".
Meanwhile, Senate Majority Leader Mitch McConnell said Mr McCain was a "hero to our country".
"He has never shied from a fight, and I know that he will face this challenge with the same extraordinary courage that has characterized his life," he said on Twitter.
Former President Barack Obama tweeted: "John McCain is an American hero and one of the bravest fighters I've ever known. Cancer doesn't know what it's up against. Give it hell, John."