The debate over how to allocate the scarce resource of donated kidneys is unfolding on a national scale.

With little public scrutiny, transplant doctors and administrators are discussing who should receive life-extending kidneys -- and support is growing for a national system that would favor the young.

The United Network for Organ Sharing, which oversees transplantation for the federal government, has already given patients younger than 18 an advantage. Last year, it moved them to the front of the line for high-quality organs from donors younger than 35.

Now the network is drafting a kidney allocation scheme for adults, who account for more than 95 percent of transplants using kidneys from cadavers. A formal proposal and public hearings are expected next year. The final decision will rest with the U.S. Health Resources and Services Administration.

The re-examination is largely based on a computer analysis of data on more than 300,000 patients placed on kidney waiting lists since 1987 -- including age, race, health, body mass index, diagnosis, years on dialysis and years of survival after transplant.

The analysis shows which types of recipients should be favored in order to squeeze the maximum life out of the pool of available kidneys.

Without a change, proponents say, the system is headed for collapse.

Over the last decade, the number of people waiting for kidneys nationwide has more than doubled to about 68,500, as of last week. The average wait now exceeds three years; up to seven years in the largest cities.

The growth has been driven by patients over 50, and increasingly over 65, who are joining the waiting list in record numbers and claiming an unprecedented share of kidneys. The population is aging, and as transplantation becomes more routine, older people are pushing for the better quality of life it can offer. At the most extreme, a hospital in Pennsylvania recently put a kidney from a cadaver into a 90-year-old.

But long before their new kidneys wear out, many older recipients die of the afflictions that come with aging. From a statistical standpoint, kidneys are being squandered.

Last March, Dr. Ben Vernon, a transplant surgeon in Denver who serves on the board of the national organ transplant network, was moved to speak out about kidney allocation, warning against the cold calculations of utilitarianism.

He invoked the U.S. sterilization of mental patients in the 1920s. Supposedly scientific arguments, he said, were "used by Nazi Germany to institute some laws that now, clearly, in retrospect, were atrocities and took them down the path toward genocide," he said.