New White House budget figures show a program to help seniors pay for prescription drugs will cost the government some $320 billion more than projected.
Figures released late Tuesday raised the estimate of the program's cost to $720 billion over the next decade.
That is nearly twice the $400 billion estimate President Bush offered in 2003, before Congress added the prescription drug benefit to the government's Medicare medical insurance program.
The increased cost is likely to draw criticism from lawmakers concerned about the soaring U.S. budget deficit.
Speaking with reporters at the White House Wednesday, Mr. Bush said Medicare has "unfunded liabilities" that he and Congress will have to deal with after fixing the Social Security retirement program.