WASHINGTON — Deaths from AIDS in the United States last year fell significantly for the first time since the AIDS epidemic began in the early 1980s, federal health officials reported Thursday.

The decline in AIDS deaths occurred in all regions of the country and in all racial and ethnic groups. However, the trend was not seen among women or among people infected with HIV through heterosexual contact — two demographic groups in which the epidemic is still growing.

Epidemiologists at the Centers for Disease Control and Prevention, which made the announcement, believe deaths from AIDS is falling for two reasons. The number of infected people who are progressing to AIDS — the advanced, often-lethal stage of the disease — is leveling off. At the same time, better medical therapies are prolonging the survival of patients who are already at that stage.

The total number of deaths from AIDS in the first six months of 1996 was 22,000, compared to 24,900 deaths during a similar period in 1995 — a 13 percent decrease, according to the data compiled by the CDC. Although there had been slight declines for short periods earlier in the epidemic, last year’s was by far the largest.

The trend appears to have begun in 1995. Only some of the fall can be attributed to the growing use of protease inhibitors, a potent new class of antiviral drugs that didn’t become widely available until last spring. Protease inhibitors are now commonly used in combination with two other antiviral drugs in what’s become known as “triple therapy.”

“AIDS deaths began to plateau in 1995, and that really suggests that something began to happen before protease inhibitors were licensed by the FDA [Food and Drug Administration],” said John W. Ward, CDC’s chief AIDS epidemiologist.

Two forces, in particular, appear to have preceded the arrival of the first protease inhibitor in December 1995. One was the use of two-drug antiviral combinations, which prolonged survival among AIDS patients even though they are less effective than triple therapy. The second is more widespread use of an anti-infective pill that helps prevent Pneumocystis carinii pneumonia, the most common “opportunistic” infection in AIDS patients, whose immune systems are severely damaged.

The decline in nationwide deaths from AIDS reflects a trend detected in several cities in the last few months.

Daily deaths from AIDS in New York fell by about 50 percent between November 1995 and November 1996. Total AIDS deaths in King County, Wash., which includes Seattle, fell by 43 percent last year compared to the average annual number of deaths in the preceding three years. In San Francisco, total AIDS deaths fell 15 percent between the last half of 1995 and the first half of 1996.

The drop in mortality nationwide was not evenly distributed among groups of AIDS patients.

Although the decline was 13 percent overall, it was 32 percent among American Indians and Alaskan natives; 21 percent among non-Hispanic whites; 10 percent among Hispanics; 6 percent among Asians; and 2 percent among non-Hispanic blacks. AIDS deaths fell 15 percent among men, and rose 3 percent among women.

AIDS deaths among men infected by sexual contact with other men fell by 18 percent. Deaths in people exposed through intravenous drug use fell by 6 percent. People exposed through heterosexual activity had a 3 percent rise in deaths.