Mayor Anthony A. Williams announced yesterday that when the District starts registering new HIV cases, they will be identified by numbers instead of names, a decision praised by members of the gay community and the D.C. Council.

Williams (D) said he chose the "unique identifier system" to protect the privacy of people getting tested, despite arguments made by some of his public health advisers and the U.S. Centers for Disease Control and Prevention that names reporting yields the most accurate picture of an epidemic whose face has changed dramatically.

The system will begin operating in the next year, after city health officials travel to states using similar systems to study how their programs work.

"This system will assure confidentiality and not discourage anyone from seeking testing," Williams said.

The debate over how best to collect data on residents infected with the AIDS virus has resulted in different systems in the Washington area. Maryland officials have been passionate advocates of the "unique identifier" system since they adopted it in 1994; Virginia has required doctors and laboratories to report the names of everyone who tests positive for HIV.

The District has the highest rate of new AIDS cases per capita in the nation, said Williams, who announced his decision at La Clinica Del Pueblo, a clinic in Columbia Heights that treats AIDS patients. The head of the clinic supported the mayor's plan. He gave his speech in Spanish, which was then translated into English. Williams tried his hand at a few lines in Spanish.

According to national data, the disease is growing fastest among Latinos, African American women, youths and heterosexuals in general.

The District has not begun to collect HIV data. An estimated 14,000 to 17,000 D.C. residents live with HIV or AIDS. AIDS remains the leading cause of death among District residents 25 to 44 years old. Last year, there were 186 cases of AIDS per 100,000 District residents--more than nine times the national average.

"It's clear to me that we have an epidemic," Williams said. "We need reliable data and a reliable tracking system."

In discussions with the mayor, the city's AIDS chief, Ronald Lewis, wanted Williams to reconsider his 1998 campaign statements favoring the numbers system. Lewis argued that the only way to avoid mistaken assumptions about who has contracted the virus and how is to list by name.

Lewis said yesterday that he wasn't "pushing" the names system but rather wanted the mayor to know that it is considered the most efficient, cost-effective method from a public health official's point of view.

But members of the D.C. Council and civil rights groups said a names system could worsen the epidemic by scaring large numbers of people away from being tested because of privacy concerns.

"There is every reason to believe that if someone thinks their name will be public in connection with this diagnosis, she or he will decide against being tested," D.C. Council member Jim Graham (D-Ward 1), former director of the Whitman-Walker Clinic, the region's largest AIDS service agency, said in a statement.

"The fact that the mayor of the nation's capital has taken such an outspoken position is very significant in this whole debate of HIV names reporting, nationwide," Graham said in an interview yesterday. "The CDC has been leaning very heavily on jurisdictions to do names reporting, and state after state has been falling in line. I can think of no other governor or other top state executive who has come out visibly in opposition to names reporting."

Only a few states use the number identifier system.

Fred Dillon, the state policy director of the San Francisco AIDS Foundation, which favors number identifiers, said a bill to put such a system in place in California was vetoed last year by then-Gov. Pete Wilson. Legislation to create such a system will come up again this fall before the state legislature.

In the District's new system, each case will be filed under a code number, based partly on the patient's birth date and Social Security number. Advocates for this method argue that this will shield identities while allowing officials to complete counts and devise prevention and education strategies.

Supporters of names reporting had argued that such data are subject to flaws, among them duplications when infected people are counted twice under two different IDs. And observers point out that no system is absolutely immune to a breach of privacy.

Williams announced his decision one day after appointing Ivan C.A. Walks, 42, a neuropsychiatrist and executive with a managed-care company, as his new public health director.

D.C. Council member David Catania (D-At Large) called Williams's decision "courageous."

"Nothing is more more intensely personal than one's health," Catania said.