"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 intravenous 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 overall 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 intravenous drug users, who can get 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 roughly stable from 2001 to 2003 but climbed 8 percent between 2003 and 2004. That was true for men of all races, CDC officials said. But 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 AIDS virus is spread through voluntary behavior. An unlimited supply of needles will not alter behavior patterns of irresponsible and often psychotic addicts," the conservative Family Research Council said in a statement.
The government does not know exactly how many people have HIV. Roughly 25 percent of people living with HIV do not know they are infected, health officials said.
The study for the first time includes data from New York, which accounted for more than 20 percent of the diagnoses seen in the 33 states. "The inclusion of New York data gives us more representative picture what going on," Lee said.
California and Illinois are among the states still missing from the database.
By Mike Stobbe