In October, the senator underwent surgery for a partially blocked artery in his neck.
Kennedy is the last surviving brother of John F Kennedy, the former president who was assassinated on November 22, 1963.
His brother Robert Kennedy was assassinated on June 5, 1968, as he was campaigning for the presidency.
A critic of George Bush, the current US president, Kennedy is a supporter of Barack Obama, who is running for the Democratic nomination to contest a presidential election in November.
First elected to the senate in 1962, he is the second longest-serving senator after Robert Byrd of West Virginia.