Republican Sen. Mark KirkMark KirkNBA pulls All-Star Game from NC over bathroom law GOP groups scale back support for Sen. Johnson Top GOP senator: Trump will have little effect on Senate races MORE (Ill.) was hospitalized Saturday after experiencing a stroke, his office said Monday.
Kirk's office said he suffered the stroke Saturday and underwent surgery on Monday to reduce swelling in his brain.
"Due to his young age, good health and the nature of the stroke, doctors are very confident in the Senator's recovery over the weeks ahead," his office said.
An ischemic stroke occurs when a blood clot obstructs a blood vessel to the brain. It is usually caused by fat or cholesterol building up along artery walls, according to the National Institutes of Health.
One of Kirk's doctors, neurosurgeon Richard Fessler, predicted the senator would not suffer any permanent mental damage from the stroke. Fessler said a four-inch by eight-inch part of the Illinois senator's skull was removed to relieve pressure.
But Fessler was less optimistic about Kirk's chances at a full physical recovery, "particularly on the left side of his body," where he is likely to lose some motor functions.
Fessler said that Kirk's recovery would take time.
"It's not going to be days," he said.
Senate Majority Whip Dick DurbinDick DurbinOpioid package clears key Senate hurdle Overnight Healthcare: Feds defend ObamaCare's affordability DNC chief spared in Sanders-Clinton talks: report MORE (D-Ill.) said he was "stunned" by the news about his friend and colleague.
"[Kirk] is young and in good physical condition and I have no doubt he will make a speedy recovery," Durbin said in a statement. "I have reached out to his staff and offered to do anything I can to help with his Senate duties. Loretta and I will keep Mark and his family in our prayers.”
Kirk was elected to the Senate in 2010, winning the seat that was formerly held by President Obama. He has focused much of his attention on foreign affairs since coming to the Senate, recently co-authoring an amendment that would impose additional financial sanctions on Iran.
In December, Kirk endorsed Mitt Romney for president.
"I am extremely distressed by the news that my friend Mark Kirk is hospitalized for emergency medical treatment," Romney said in a statement. "I wish him a speedy recovery and a swift return to the U.S. Senate chamber, so he can continue his important work for the people of Illinois and all the people of the United States."
— This story was last updated at 1:39 p.m.