The use of Social Security numbers for personal identification purposes has been gradually reduced by state and federal agencies and health insurance companies turning to other means. But the federal government continues to use them for Medicare purposes and risk exposing senior citizens to identity theft despite repeated warnings about the danger.

Last week, the inspector general of Social Security, Patrick O'Carroll Jr., called for the Centers for Medicare and Medicaid Services to find an alternative to the present system which puts personal information at risk. The Social Security numbers appear on Medicare cards 40 million beneficiaries carry with them and can give identity thieves access to other data.

Blue Cross and Blue Shield have stopped using the numbers for more than 100 million people covered by their plans nationwide. Byron Hollis, director of the company's anti-fraud department, said that "medical identity theft is the fastest-growing form of health care fraud."

Thirty-one states have laws that prohibit or restrict their use, including on public records that are easily accessible online. Even other federal agencies, including the Department of Veterans Affairs, have stopped using Social Security numbers for identification purposes. The Defense Department plans to issue 8 million new identification cards in the next few years that will have just the last four digits of the number.

Yet, Medicare services resists making any adjustments as too costly and even alarming to beneficiaries. Federal officials estimate it would cost about $500 million to change their computer systems.

The agency's resistance runs contrary to advice in a White House Office of Management and Budget memo from May 2007 that said the federal department should develop plans "to eliminate the unnecessary collection and use of Social Security numbers within 18 months."

Charlene M. Frizzera, chief operating officer of the Centers for Medicare and Medicaid Services, said it would take at least three years of planning and another eight years to implement a change. However, Mr. O'Carroll's report is not the first to raise concerns. They have been heard for years. Legislation has been proposed to ban their use. Medicare officials are right that challenges exist, but other governments, public entities and private agencies have been able to make the transition away from Social Security numbers.