The number of HIV infections remains stable in the United States, but American health officials say the situation may hide alarming increases among racial minorities and homosexual men.

The latest figures from the U.S. Centers for Disease Control confirm that the nation's AIDS cases and HIV infections have remained level since 1998.

That would be good news in many countries where the epidemic is expanding. But the head of the CDC's HIV Prevention office, Ronald Valdisseri, told the International AIDS Conference in Barcelona that the number of new U.S. cases had been declining substantially in the two years before 1998, thanks to potent new AIDS drugs. "Progress toward further reductions in HIV has stalled in recent years," he said. "An estimated 40,000 Americans continue to become infected every year with HIV."

Dr. Valdiserri says the lack of continued progress against AIDS in the United States is probably the result of several factors. He suggests that a substantial number of infected Americans, as many as half of the one million who have the virus, either do not know of their condition or are not getting medical care. Other possible causes are treatment failure and the difficulty many patients have following complex treatment regimens that often cause severe side effects.

But the notion of a stable U.S. HIV epidemic is deceptive. The CDC numbers show certain populations where the impact is increasing.

Nearly half of the new HIV diagnoses between 1994 and 2000 were among homosexual men. The infection rate in this group was about nine times higher than in the heterosexual population.

But heterosexual rates have been growing, too, though more slowly. They have risen 10 percent in that seven year period. The impact is disproportionately large among blacks, who accounted for 75 percent of the new heterosexual infections.

The problem could be even worse than the statistics reveal. The CDC data account for only one-fourth of estimated new HIV infections in the United States. They exclude half of the 50 states, such as the populous California, New York and Florida, which have substantial minority and homosexual populations.

Dr. Valdiserri suggests that complacency has set in about HIV, with a loss of the sense of urgency and concern that characterized the first decade of the epidemic in the 1980s. "We are very concerned about increases in risky behavior, and we are poised at a critical time in HIV prevention efforts in the United States," said Dr. Valdiserri. "If we do not strengthen programs, if we do not recommit to HIV prevention, there is a real possibility that we could see increases in HIV."

The most positive news in the American HIV situation is that about infected newborns. A separate Centers for Disease Control study shows that they declined 80 percent in the past decade. Roughly 300 U.S. babies were born with the virus in 2000, compared to a peak of nearly 1,800 in 1991.