Sen. Mark Kirk was hospitalized after suffering a stroke over the weekend and underwent surgery Monday in Chicago, his office said.
According to a statement, Kirk, 52, checked himself in to Lake Forest Hospital in the Chicago area on Saturday, where doctors discovered a carotid artery dissection in the right side of the neck.
After being transferred to Northwestern Memorial Hospital in Chicago, further tests showed the senator had suffered an ischemic stroke.
On Monday, Kirk underwent "successful" surgery to relieve swelling around his brain that resulted from the stroke, the statement continues.
"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," Kirk's office said.
His doctors are expected to provide more details at a news conference shortly.
In Kirk's office in the Hart Senate Office Building, where the phone lines rang non-stop, an aide to the senator released a paper copy of the statement and said no spokepeople were available for further questions.
Kirk captured President Obama's old U.S. Senate seat in a close 2010 election against Democrat Alexi Giannoulias.
His victory capped a tumultuous political fight that began when former Gov. Rod Blagojevich was charged with attempting to sell the Senate seat for personal gain – a charge he was convicted of last year.
During 10 years in the U.S. House before his Senate election, Kirk has forged a reputation as a centrist Republican.
He has sought to maintain that reputation amid the bitter partisan atmosphere in Washington, including working with Democratic Majority Leader Dick Durbin, Illinois’ senior U.S. senator. Kirk recently endorsed Mitt Romney for the Republican nomination for president.
Kirk, who lives in Highland Park, Ill., is divorced and has no children.
The Chicago Tribune contributed to this report.