Senator Edward "Teddy" Kennedy, 76, is the most prominent living member of the Kennedy family, and the last of the brothers who dominated US Democratic politics in the 1960s.
"Teddy" Kennedy has been a senator for more then four decades
Three infamous tragedies marked his life in that decade - the assassinations of his brothers John and Robert, and a fatal car accident on Chappaquiddick Island in 1969, which clouded his own political prospects.
But as a member of the Senate since 1962 - re-elected seven times - he has become one of the Democratic Party's best-known figures and a leading reform campaigner.
Lately he has been an active supporter of Barack Obama for the Democratic presidential nomination.
"Ted Kennedy is a giant in American political history," said Mr Obama on hearing of his ill-health on 17 May.
Last of the brothers
The man who represents Massachusetts in the Senate was born in Boston on 22 February 1932, one of nine children to Irish-American businessman and politician Joseph P Kennedy and his wife Rose Fitzgerald.
Educated at Harvard University and the University of Virginia Law School, he was elected to the Senate to fill the vacancy caused by the resignation of his brother John on becoming president.
The eldest of his three brothers, Joseph, had died first, killed in a World War II air crash. In 1964, Edward himself survived a plane crash in which a pilot and one of his aides were killed.
In 1963, John was shot by an assassin in Texas. Less than five years later, Robert was dead, the victim of a gunman in Los Angeles.
Edward was to try for the presidency himself: first in 1974 when he ultimately withdrew his candidacy and then in 1980, when he failed in a primary challenge to Jimmy Carter.
Correspondents say that his presidential ambitions were haunted by an accident at the Massachusetts island of Chappaquiddick in 1969, when his car plunged off a bridge and a young woman
riding with him, Mary Jo Kopechne, drowned.
He pleaded guilty to leaving the scene of a crime and was given a two-month suspended jail sentence.
After more than 40 years of service as a senator, Edward Kennedy has earned a reputation as both a liberal lawmaker and a dealmaker able to bridge party lines, correspondents say.
Sen John McCain, the likely Republican presidential nominee, paid tribute to his political foe on hearing of his health problems.
"Senator Kennedy's role in the US Senate cannot be overstated," he said.
"He is a legendary lawmaker and I have the highest respect for him. When we have worked
together, he has been a skilful, fair and generous partner."
His current positions include senior Democrat on the Health, Education, Labor and Pensions Committee and service on the Judiciary Committee, the Armed Services Committee and the Congressional Joint Economic Committee.
Also a member of the Congressional Friends of Ireland, he is a trustee of the John F Kennedy Center for the Performing Arts in Washington DC.