Tests revealed a malignant tumor in McCain's skull following a surgery to remove a blood clot above McCain's left eye that took place last week.
McCain is being relied on by US President Donald Trump and other Republicans to help push the "repeal" or "repeal and replace" bill through Congress to end Obamacare, but it's no longer clear if he will be able to faithfully carry out his duties in office with a clear head.
"Tissue pathology revealed that a primary brain tumor known as a glioblastoma was associated with the blood clot," was found in McCain's brain, according to the hospital's statement.
Chemotherapy and radiation therapy are being considered by McCain and his family. McCain’s doctors said he is recovering well from the surgery and his "underlying" health is excellent.
"Senator McCain received excellent treatment at Mayo Clinic Hospital in Phoenix, and appreciates the tremendous professionalism and care by its doctors and staff," the senator's press office said Wednesday.
"He is in good spirits and recovering comfortably at home with his family. On the advice of his doctors, Senator McCain will be recovering in Arizona next week," the announcement added.
The American Brain Tumor Association reports that "for adults with more aggressive glioblastoma, treated concurrently with temozolamide and radiation therapy, median survival is about 14.6 months and two-year survival is 30 percent." A 2009 study found that "almost 10 percent of patients with glioblastoma may live five years or longer."
McCain was the Republican nominee for president in 2008. He wound up losing to Barack Obama by an electoral vote count of 365-173.
The 80-year-old was first elected to the Senate in 1987 and has been considered one of the most influential voices in the legislative body for more than a decade. He ran for president in 2000 before losing the nomination to George W. Bush, who went on to become president from 2001 to 2009.