NAPLES, OCT. 9 -- Children comprise as many as one-third of all AIDS cases in some African countries, a situation that threatens to wipe out reductions in infant mortality of recent decades, according to health officials attending the Second International Symposium on AIDS in Africa, which ended today.

The conference, whose sponsors include the World Health Organization, also heard reports contradicting common ideas about AIDS in Africa, which has the greatest estimated incidence of acquired immune deficiency syndrome in the world.

Delegates reported that common use of needles is not a factor in the spread of the virus there, that the disease is not spread widely over the continent but remains concentrated largely in local pockets centering on cities and connecting truck routes, and that there is evidence hinting that the AIDS virus did not originate in African monkeys.

The two-day conference was marked by a new openness on the part of previously reluctant African governments, who freely discussed the disease and the rapid progress in some countries in building national AIDS programs.

Rwanda, one of the countries with a new program that includes a recently established blood screening effort, reported that 35 percent of all its officially reported cases of AIDS are in children.

Dr. A.A. Ndikuyeze, an epidemiologist with the Central University of Public Health in Butare, Rwanda, reported that the country has two widely separated peaks in the profile of its AIDS victims -- one below age 3 and the other in the prime years of sexual activity between the ages of 20 and 40.

Other central African countries such as Zambia and Zaire also have reported that more than 20 percent of their AIDS cases involve children.

Delegates said the epidemic among children stems from two factors of the illness in Africa: as many women are infected as men, and African women have the greatest number of children per capita in the world.

Dr. Nathan Clumeck of the Saint Pierre Hospital in Brussels said that among his African AIDS patients he has frequently found that it is as distressing for a woman to hear she should not have children as that she has AIDS.

"It is the major goal in life for many African women. And if they do not have children, they may be rejected by their husbands and their villages," he said.

Jonathan Mann, director of the World Health Organization's program on AIDS, said, "The projected gains in infant mortality through the massive programs of recent decades may be cancelled by the exponential rise of AIDS among children in some African countries."

African researchers also have been concerned that reuse of needles might be a major risk factor in passing AIDS-infected blood from patient to patient. For health reasons, children are among the most frequently injected.

A shortage of syringes has made reuse of needles common among doctors in some countries, as well as by local "injectionists" -- unlicensed local healers -- who use unsterilized needles repeatedly in treating a wide variety of illnesses. But several researchers here reported that injections appear to offer little risk.

Dr. Bosenge Ngaly, a researcher at the Mama Yemo Hospital in Kinshasa, Zaire, reported that in a survey of the hospital's pediatric emergency ward, there was no difference in the number of injections received by children infected with AIDS and those not infected.

He said the chief means of infection among the infants was to receive it either in the womb or at birth from an infected mother.

On an issue that brought angry outbursts at the last conference on AIDS two years ago in Brussels, Dr. Luc Montagnier of the Pasteur Institute in Paris said, "The evidence does not support a conclusion that AIDS had its origin in Africa."

Though the place of origin is still unknown, Montagnier said that there are a few pieces of evidence that argue against the origin of the virus in monkeys in Africa, as western scientists have suggested.

He said a study of the prevalence of the virus among pygmies who commonly eat African green monkeys, the type suspected of being the origin of AIDS, showed virtually no AIDS infection. If the virus originated among the monkeys, the pygmies should have been the first infected, he suggested.

He also said that if AIDS originated in Africa, it would be much more likely to have spread first to Europe rather than to the United States. The first AIDS epidemics occurred in the United States and Africa nearly simultaneously around 1980, researchers said.

Dr. Gottlieb Monekosso, director of the World Health Organization's African office, added that the question of origin should not be emphasized now.

"When a fire is burning in your house, it's not the time to start asking who lit the match or who was smoking the cigarette," Monekosso said. African leaders were upset in previous years when westerners "blamed AIDS on Africa."