Killebrew, 74, said in a statement issued Thursday by the Minnesota Twins, that while his "condition is very serious, I have confidence in my doctors and the medical staff and I anticipate a full recovery."
Killebrew said he is being treated at the Mayo Clinic in Rochester, Minn.
"With my wife, Nita, by my side, I have begun preparing for what is perhaps the most difficult battle of my life," his statement said.
Killebrew played 22 seasons in the major leagues, starting in 1954 with the Washington Senators and moving with the team to Minnesota in 1961, where he played for the Twins through 1974. He also played one season for the Kansas City Royals before retiring after the 1975 season.
He was voted the American League most valuable player after the 1969 season in which he led the lead with 49 home runs and 140 runs batted in.
Killebrew, who was voted into the Baseball Hall of Fame in 1984, played in 2,435 games, batting .256 and knocking in 1,584 runs. His home run total of 573 ranks 11th on the major leagues' all-time list.