The International AIDS Conference has opened in Spain with the largest attendance of the 14 such meetings held so far since the 1980s.

At least 17,000 people have descended upon Barcelona, 15,000 of them conference delegates, the rest journalists and conference staff. They gathered Sunday evening at a time when the United Nations says the AIDS pandemic is exploding, becoming the fourth leading cause of death worldwide.

The theme of this year's conference is knowledge and commitment for action. Conference officials say both have grown since the last meeting in Durban, South Africa two years ago, leading to hope that the spread of the disease can finally be slowed.

In the realm of knowledge, the conference co-president, University of Barcelona medical professor, Jose Maria Gatell, said scientists have submitted more than 10,000 scientific papers, including studies on vaccines and two new families of drugs. He said, "This is a very relevant issue because the last family of compounds was presented six years ago. We have introduced some new medicines belonging to the same families, but here we are going to discuss two completely new families of compounds."

Dr. Gatell says he hopes the conference advances the drive for affordable medicines that are easier to use than existing ones.

As for commitment for action, the other leg of the AIDS conference theme, United Nations AIDS Program Executive Director Peter Piot cites an increase in high-level political attention to the epidemic since the Durban meeting and a six-fold increase in money directed at it.

But he said the political will and funding levels are still insufficient. "It is the lack of action," he said. "That is the main reason why we have an expanding epidemic, and it is a failure of leadership, the failure of recognizing that HIV is a threat to the survival of countries."

Dr. Piot hopes the conference mobilizes people to work towards eradicating HIV.

But at least one participant is uncertain about that outcome. Shaun Mellors of South Africa is an HIV-positive member of the conference organizing board. He thinks 15,000 delegates are a sign these biennial meetings are getting bloated rather than more dedicated. Mr. Mellors said, "Some of us are concerned that these conferences are getting too big. Often what happens at these conferences is that you have high-powered speeches, high-powered politicians, high-powered photo opportunities during the week of the conference. After the conference, we do not see effective action, any effective implementation until the next international AIDS conference, and it cannot be allowed to continue like that."

But another conference organizer, International AIDS Society President Stephano Vella, said the meeting's strength is its large participation, its openness to all and lack of control by governments with a political agenda. "The governments have come," he said, "and they are welcome. "But these conferences have been led by the community and scientists, and this is the strength of these conferences because we are free to speak. We are free to increase awareness. Look at what happened in Durban and what will happen here. This is the most successful conference organized in terms of attendance."

Mr. Vella says he expects this International AIDS Conference to have what he calls an incredible impact. But he notes that while the Durban meeting two years ago helped break official silence about HIV, he concedes that to be successful, the present gathering must incite more action.