The president and Mrs. Bush are praying for Pope John Paul II, who the Vatican says is in very serious condition.
White House spokesman Scott McClellan says the president and Mrs. Bush join people around the world in prayers for the Holy Father.
He says the outpouring of love and concern from so many, including millions of Americans, is a testimony to the pope's greatness.
Mr. McClellan says President Bush is being kept up to date on the pope's health by White House Chief of Staff Andy Card, and the U.S. Embassy to the Holy See is in close contact with the Vatican.
The president and the pope last met in Rome in June, when Mr. Bush awarded the pontiff America's highest civilian honor for his moral leadership.
Vatican officials say the 84-year-old pope remains lucid, fully conscious and serene. He had a high fever Thursday from a urinary tract infection. Officials say he has been told of the gravity of his situation, but wants to stay in his residence at the Vatican, instead of returning to hospital.
By Scott Stearns
White House