A spokesperson for Sen. Mark Kirk says the Illinois Republican has suffered a stroke and has undergone surgery.
The 52-year-old Kirk checked himself into Lake Forest Hospital in Illinois over the weekend. He was later transferred to Northwestern Memorial Hospital in Chicago, where tests showed that he had suffered a stroke.
He underwent surgery early Monday morning to relieve swelling around his brain.
A statement from Kirk's office said the surgery was successful and that doctors are "very confident" in his recovery based on his age and health.
Kirk was elected to the Senate in 2010, winning the seat formerly held by President Barack Obama.
The hospital plans a news conference this morning.