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Alex Brandon, File, Associated Press
FILE - In this Dec. 16, 2010, file photo, Sen. Mark Kirk, R-Ill., talks to the media on Capitol Hill in Washington. Kirk's office says he has suffered a stroke, and is undergoing surgery.

WASHINGTON — A spokesperson for Sen. Mark Kirk says the Illinois Republican has suffered a stroke and has undergone surgery.

The 51-year-old Kirk checked himself into Lake Forest Hospital in Illinois. 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 later today.