The rate of newly reported HIV cases among blacks has been dropping by about 5 percent a year since 2001, the government said Thursday, but blacks are still eight times as likely as whites to be diagnosed with the AIDS virus.

"The racial disparities remain severe," said Lisa Lee, an epidemiologist at the Centers for Disease Control and Prevention.

The falling rate among blacks seems to be tied to overlapping drops in diagnoses among injection drug users and heterosexuals, CDC researchers said.

The study was based on 2001-04 data from 33 states that have name-based reporting systems for HIV. Health officials do not know which diagnoses represent new infections and which ones were infections people had for years but had just discovered.

The CDC found that annual number of diagnoses in the 33 states decreased slightly, from 41,207 cases in 2001 to 38,685 in 2004. The rate fell from 22.8 cases per 100,000 people in 2001 to 20.7 per 100,000 in 2004.

The decline was more pronounced among blacks: The rate dropped from 88.7 per 100,000 in 2001 to 76.3 in 2004. Among whites, the rate rose slightly from 8.7 to 9.0.

At least part of the decline among blacks appears to be tied to a 9 percent annual decline in diagnoses among injection drug users, who can contract the virus from contaminated needles. More than half of the drug users were black, Lee said.

The decline is also linked to a 4 percent decline in diagnoses among heterosexuals. About 69 percent of the heterosexuals diagnosed with HIV were black.

Diagnoses among men who have sex with men remained stable from 2001 to 2003 but climbed 8 percent between 2003 and 2004. That was true for men of all races, CDC officials said. They said they could not explain the recent increase.

In New York, needle exchange programs helped explain declining HIV infection rates, said state Health Department spokeswoman Claire Pospisil. New York introduced needle exchanges in 1992, and 114,500 people have participated, she said.

Most public health researchers say such programs have been clearly effective against the spread of HIV, but some argue they work against efforts to fight drug abuse.

The government does not know exactly how many people have HIV. About 25 percent of people living with HIV do not know they are infected, health officials said.