The U.S. government will send only one-quarter as many people to the huge international AIDS conference starting Sunday in Bangkok as it sent to the last one in Barcelona.

The decision to cut attendance, which comes as the Bush administration is rolling out its five-year, $15 billion global AIDS treatment plan, was reached long after many government scientists had made plans to attend the conference, which is held every two years. Dozens of scientific presentations were withdrawn, about 50 will be published only as summaries and not presented publicly, and dozens of meetings -- many designed to train Third World AIDS researchers and foster international collaboration -- were canceled.

The move, which officials say is to save money, is interpreted by many AIDS experts as payback for the heckling of Health and Human Services Secretary Tommy G. Thompson at the last AIDS conference and further evidence of a "go-it-alone" attitude in the administration's global AIDS program.

The cutbacks affect only the HHS, home to the National Institutes of Health and the Centers for Disease Control and Prevention -- the two powerhouses of U.S. AIDS research. In 2002, the department spent $3.6 million on the AIDS conference and sent 236 people. This year it will spend $500,000 and send 50.

Other Cabinet-level departments with AIDS programs -- Defense, Veterans Affairs and State -- are not cutting back their participation in the Bangkok conference.

"We received no directions to do that. In fact, we received the opposite," said Col. Deborah L. Birx, an Army AIDS researcher. "Because our work is primarily international, our general encouraged us to extend the relationships."

William A. Pierce, spokesman for the HHS, said the department's decision reflected a policy of reducing travel to scientific meetings "put in place quite a while ago. This is not exclusive to this conference -- this is for all international conferences. A lot of it was simply looking at expenses." The NIH and the CDC were notified in March of the limits on their Bangkok delegations.

The U.S. government's diminished presence is being greeted in some quarters with chagrin, amazement and disgust.

"It's unfair, it's a pity, it's also a bit awkward," said Joep Lange, a Dutch scientist who is president of the International AIDS Society and a co-chairman of the conference.

"The largest group in the world in terms of AIDS expertise comes from the U.S., so it's important this expertise is at the conference," said Peter Piot, head of UNAIDS, the program run by the United Nations and the World Bank. The reduced attendance "is a big deal for the quality of the conference," he said.

Nils Daulaire, a former official of the U.S. Agency for International Development who now heads a Washington-based advocacy group called the Global Health Council, said "there's a shibboleth that these [conferences] are junkets, but they are not. They are intensive and hard work. You don't get many opportunities to get the critical mass that you do at a meeting like this. I always come out . . . having learned something that I hadn't even thought about."

This year's conference is expected to be especially large -- as many as 20,000 participants -- and important because of major new efforts to bring AIDS treatment to the Third World. The biggest of those is the President's Emergency Plan for AIDS Relief (PEPFAR), which aims to treat 2 million people with antiretroviral drugs, and prevent 7 million HIV infections over the next five years.

Despite its size and ambitions, PEPFAR has been criticized. Complaints include the decision to limit aid to 15 named countries; the promotion of AIDS-prevention strategies that emphasize abstinence; and the requirement that foreign-made generic drugs used in the program must be approved by the Food and Drug Administration.

Many people urging greater U.S. commitment to combating AIDS globally -- including numerous members of Congress -- also believe PEPFAR should give more money to the Global Fund to Fight AIDS, Tuberculosis and Malaria, an entity created 21/2 years ago at the suggestion of U.N. Secretary General Kofi Annan. The Bush program is committed to giving $1 billion of the $15 billion in PEPFAR money to it.

The decision to limit U.S. participation in the Bangkok conference is sending a message the rest of the AIDS world will not miss, said a senior CDC official who declined to be identified.

"It's a perception from the rest of the world that the U.S. wants to be engaged, but the U.S. wants to call the shots," the official said.

The decision has caused consternation at the CDC and the NIH and among AIDS scientists outside the government whose work is funded by those agencies. Almost nobody was willing to speak on the record because of fears of retaliation.

"What can I say? I can't say anything," an anguished NIH researcher said.

Calling the decision "inappropriate and misguided," one AIDS scientist said that for the NIH staff "it is quite demoralizing to get an abstract accepted in the field of your choice, and then not be able to present your findings because you're not allowed to go the meeting."

Jack Whitescarver, director of the NIH Office of AIDS Research, declined to be interviewed. The NIH released details of the cutbacks only in response to a Freedom of Information Act request.

The records show that 30 people from the National Institute of Allergy and Infectious Diseases alone canceled plans to attend. Seven research reports -- including lectures and poster presentations in which scientists stand by a display of their research and answer questions -- were withdrawn.

Workshops on "laboratory quality assurance" and "sustainable treatment of HIV/AIDS" were canceled. So was one at which NIH officers would have counseled Third World scientists on how to apply for grants from the agency's international program -- an increasingly important part of the NIH AIDS research portfolio. The National Institute of Mental Health canceled a "satellite" conference on the use of the Internet to deliver HIV-prevention messages.

Some of the meetings between NIH researchers and foreign collaborators, scheduled during the Bangkok meeting, will have to be held later, officials said.

The CDC, the other major federal agency involved in AIDS research and prevention, is sending 20 staff members from the United States and a half-dozen from Southeast Asia to Bangkok, compared with 90 who went to Barcelona in 2002. In previous AIDS conferences, the CDC sent 68 people to South Africa, 80 to Geneva, 157 to Vancouver, about 30 to Japan and 50 to Berlin.

Ronald O. Valdiserri, the epidemiologist leading the CDC's delegation, said 46 poster presentations and one lecture were withdrawn. Twenty-nine lectures will be delivered, although in many cases not by the primary investigator. Of the 148 CDC posters still on the schedule, 40 will be published only as summaries in the abstract book and not displayed. A satellite symposium on the use of rapid HIV tests was also canceled.

HHS officials tried to cancel a $250,000 CDC grant to the conference for scholarships for Third World AIDS researchers, said a person familiar with agency. When told the money could not be reclaimed, Thompson's office stipulated it go only to scientists in the countries getting aid under the PEPFAR program.

The NIH, which in the past gave grants to AIDS conferences, is not this year because "it chose not to," said Pierce, Thompson's spokesman.

A CDC official labeled as "bull" the HHS explanation that the cutbacks were primarily to save money.

"This is clearly the result of the booing of Secretary Thompson in Barcelona, which he took quite personally," this person said.

Two years ago, about 30 activists heckled the secretary with shouts of "Shame, shame!" "No more lies" and "Lies, lies!" -- making his 15-minute speech inaudible. Neither of the two speakers who followed -- Richard G.A. Feachem, director of the Global Fund, and Gro Harlem Brundtland, director general of the World Health Organization -- came to Thompson's defense or criticized the protesters.

Later that day, some of the hecklers met with Thompson, told him of their concerns and urged him not to take the catcalls personally. Several insiders said, however, that some people high in the HHS viewed the jeering as a serious affront to civility, U.S. generosity and the Bush administration.

Within weeks of the conference's end, word circulated that HHS participation might be different the next time.

Protesters jeer HHS Secretary Tommy Thompson in 2002. Some say the cutback in this year's attendance is payback.