The Cape Cod Times first reported this weekend's medical emergency and has posted a photo of Kennedy being taken to a MediFlight on a stretcher.
A Democratic Senate source told ABC News at the time, "The situation is serious. We just don't know how serious."
The Kennedy family was set to host the annual Best Buddies Challenge event Saturday evening at the Kennedy family compound.
Kennedy is no stranger to brushes with death.
In 1964, only a year after the assassination of President Kennedy, the newly elected senator and Sen. Birch Bayh, D-Ind., were nearly killed in a horrific plane crash that claimed the life of the pilot and one of Kennedy's aides.
Kennedy suffered two fractured ribs and three broken vertebrae in his back, an injury that would cause him a lifetime of back pain.
In October 2007, Kennedy had surgery to repair blockage in the left carotid artery in the neck believed to be connected to the injuries he suffered in the crash more than 40 years ago.
In July 1969, a year after the assassination of his brother, Sen. Robert F. Kennedy, the senator once again escaped a fatal crash.
After a night of partying, Kennedy drove his mother's 1967 Oldsmobile over a wooden bridge on Chappaquiddick, an island off of Martha's Vineyard, killing passenger Mary Jo Kopechne.
Questions surrounding how Kennedy survived while Kopechne drowned have haunted the senator ever since.
He was convicted of leaving the scene of an accident and lost his driver's license for a year, but the Kopechne family never sued the Kennedys or the senator for his role in the incident.
In more recent years, Kennedy was traveling with the late Sen. Paul Wellstone, D-Minn., on the day Wellstone's campaign plane crashed just outside Eveleth, Minn.
Kennedy had been on several flights with Wellstone on the twin-engine turboprop but did not accompany him on the fatal flight, as Wellstone was traveling to a funeral and not a campaign event.
Wellstone, his wife, daughter, three staff members and the two pilots were killed in the crash.
Even though he is the third longest-serving senator in the history of the body -- with 45 years and two months in office, behind only the current Sen. Robert Byrd of West Virginia and Strom Thurmond of South Carolina -- Kennedy is one of the hardest-working members of Congress.
As the chairman of the Health Education and Labor Committee, he was the Democrat who ran the Senate floor last week when senators were considering a bill to give collective bargaining rights to all police and firemen. He was also the chief Democrat who fought for a recently passed student loan improvement bill.
Kennedy chaired a widely covered hearing May 12 about how to fight cancer. Among the people who testified during that hearing were Lance Armstrong, Elizabeth Edwards and Steve Case.
Kennedy was also set to play a pitvotal role for Democrats as they consider the supplemental war funding bill next week. Kennedy was one of the loudest critics of the war in Iraq from its outset.
Before his illness this weekend, Kennedy was apparently working; his office released a statement in his name, congratulating Benjamin Jealous on his election as head of the NAACP.
The Associated Press and ABC News' Jen Duck, Eloise Harper, Bret Hovell, Dean Schabner, Stephanie Z. Smith, George Stephanopoulos and Sunlen Miller contributed to this report.