WASHINGTON — President Bush plans to sign a bill next week that commits the United States to spending about $40 billion over the next five years to fight AIDS overseas, a major expansion of what many consider his most successful foreign policy initiative.

The legislation also extends an implicit pledge that has little precedent in the history of U.S. foreign assistance: to continue purchasing lifesaving drugs for millions of individual people in developing countries for an indefinite period of time.

Foreign aid for health care has traditionally been used to put up buildings, buy equipment and train workers. Direct medical care of individuals was limited to one-time interventions such as vaccinations, emergency treatment after natural disasters, and curative treatments of limited duration for diseases such as tuberculosis or leprosy.

Bush's program is fundamentally different. So far, it has purchased vast quantities of antiretroviral drugs and supported day-to-day medical care for more than 1.4 million people whose survival depends on continued treatment.

"It is the first time I can think of where we have foreign aid treating a chronic disease," said Michael H. Merson, director of Duke University's Global Health Institute and a former head of the World Health Organization's AIDS office. "It's a challenge to take this on. I think the questions it raises are going to be important ones for the future."

Once started, AIDS therapy must continue indefinitely, because stopping it can rapidly lead to death. As a consequence, international health experts and medical ethicists say it would be immoral to withdraw the financial assistance that pays for the therapy unless someone else steps in to replace it.

Although all governments and organizations supporting AIDS patients overseas have made an implied open-ended commitment, it looms largest for the United States, which provides about 40 percent of global AIDS assistance. Few experts say they think the needy countries now getting help from the United States will be able to fend for themselves anytime soon.

"We've never really been confronted with this in the international health arena," said Paul De Lay, a physician formerly with the U.S. Agency for International Development who is now with UNAIDS, a United Nations program in Geneva.

The President's Emergency Plan for AIDS Relief (PEPFAR), a surprise announcement in the 2003 State of the Union address, has spent about $19 billion in the last five years. The president sought to double it to $30 billion in a bill reauthorizing it for another five years, but Congress trumped him, sending him a larger bill that when it passed on Thursday authorized the spending of $48 billion — $39 billion for AIDS and the rest for other diseases.

The money is provided to governments, charitable organizations and academic medical centers to buy drugs and equipment, train workers and run programs. By the administration's estimates, PEPFAR in five years has provided antiretroviral drug therapy for 1.4 million people; treatment for 1 million infected women during pregnancy so they are less likely to transmit the virus to their babies; care for nearly 3 million AIDS orphans; and 33 million counseling-and-testing sessions.

Although Bush's initiative enjoys support in both parties and has been praised around the world, some policy experts say the implications of its open-ended commitment have been largely ignored. A few view PEPFAR as essentially an open-ended "entitlement program" for citizens of other countries.

One person with that view is Mead Over, a former World Bank economist who is now at the Center for Global Development, a Washington think tank. He fears that if PEPFAR's commitments grow, as they are likely to, they will squeeze out funding for other, equally important, foreign aid.

He also worries that with such lopsided and personal relationships between countries, providing life-sustaining care may become a "strategic resource that will be used, or will be an implicit bargaining chip, in negotiations." He added that "sovereign countries are likely to feel quite vulnerable if they perceive that the lives of a substantial number of their citizens are dependent on the continued largess of a donor."

Over's solution is to spend much more AIDS assistance on prevention efforts, and to channel money for treatment through international organizations to spread future obligations among as many donors as possible.

Others, including PEPFAR's director, do not see such problems.

"The notion that this is going to have a harmful effect on our relationship with other countries in the long term is exactly the opposite not only of what I believe but of what is being shown," said Mark R. Dybul, a physician and AIDS researcher who also holds the rank of ambassador at the State Department, which administers PEPFAR.

Dybul said the program is sowing goodwill at the same time it is stabilizing AIDS-ravaged countries in a way that will ultimately serve American interests. The fact that AIDS is a disease requiring lifelong treatment may even have an unexpected benefit, he says. The AIDS treatment programs are forcing permanent improvements in medical care — with spillover benefits for the whole population.

"We are building systems and capacity for country ownership in ways that has never happened before," Dybul said. "That's real development."

What is certain is that the AIDS-treatment lifeline between rich and poor countries is growing rapidly. About 3 million people in low- and middle-income countries are now on the drugs. Five years ago, the number was 250,000.

As of the end of March, Bush's program was directly underwriting the care of 1,383,300 AIDS patients. Most were in 15 "target" countries: Haiti and Guyana in the Americas, Vietnam in Asia, and 12 in sub-Saharan Africa. The care of another 344,700 people is supported indirectly by money going to improve laboratories, information systems, logistics and other improvements of health-care delivery.

The other big funder is the Global Fund to Fight AIDS, Tuberculosis and Malaria, an independent organization in Geneva that gets money from rich countries, foundations and individuals, and awards it to approved programs in low-income ones. As of last December, the fund was underwriting treatment of 1.4 million AIDS patients, more than twice as many as the year before.

The expanded U.S. program aims to have 3 million people on antiretroviral AIDS treatment by 2015. Both the U.N. General Assembly and the Group of Eight industrialized countries are on record as supporting "universal access" to AIDS treatment, defined as providing treatment to 80 percent of those who need it. That would mean 18.6 million people on daily AIDS drugs by 2015, according to an estimate by UNAIDS.

The cost is huge, even considering the recent steep decline in the price of AIDS drugs. By 2015, the price tag for "universal access" to AIDS treatment would be $19 billion a year.

When the full package of prevention services, aid to orphans, and "health systems building" is added in, the global bill for AIDS services in the developing world will be $50 billion a year. Two-thirds of that money will have to come from rich countries. This compares with $110 billion spent last year for all foreign aid by all countries of the world, of which about $10 billion went for AIDS.

Few experts think that the financial responsibilities, whatever their magnitude, will end in 2015. As patients live longer they will take drugs longer, and many will eventually need more expensive ones, too.

Studies suggest that after several years of treatment, about 5 percent of patients each year become resistant to one or more of their antiretrovirals. If 8 million people are on treatment by the end of this decade — which many experts think is realistic — about 800,000 will need "second-line" therapy, which now costs 10 times as much as the cheapest starting combination.

Although there is debate in the global health community about whether programs are obliged to switch all failing patients to second-line treatment, or even third-line "salvage" therapies, everyone agrees that first-line treatment can never be stopped.

"The train is out of the station. People who are on treatment — we can't drop them, if for no other reasons than ethical ones," said Peter Piot, the Belgian physician who heads UNAIDS.

The Global Fund already recognizes that treatment, once started, is essentially an irrevocable entitlement. If the fund cancels a country's AIDS grant for reasons of graft or incompetence, it promises to find a way for patients already on antiretroviral therapy to stay on without interruption.

Nevertheless, some experts think AIDS-treatment programs are not so different from other forms of foreign aid. Their obligations are just more obvious.

Dean T. Jamison, a health economist at the University of California at San Francisco, recalled recently that when he worked at the World Bank in the 1970s, Robert S. McNamara, the bank's president, required that staff overseeing projects in Africa demonstrate in writing how the recipient countries would shoulder the responsibility after five years.

Jamison said he and his colleagues dutifully complied, making up rosy economic scenarios that none of them believed. Privately, they all knew that whenever they drew up an assistance project, "we were putting together a set of implicit financial commitments that were likely to last for decades," he said.

And they have.