ATLANTA (AP) _ Federal health officials on Thursday reported outbreaks of hepatitis A among homosexual men, who have not historically been at high risk for the viral liver disease.

Researchers with the U.S. Centers for Disease Control said they fear the outbreaks in several cities could signal a resurgence of unsafe sex, a decade after concern over AIDS prompted changes in sexual behavior.

Hepatitis A typically enters the body orally, through lapses in hygiene that result in contact with fecal matter, such as in day-care centers or homes with children in diapers. But some forms of anal sex also could cause similar contamination, the CDC said.

From 1982 to 1989, fewer than 10 percent of hepatitis A cases occurred in homosexuals, the CDC said.

But in the first half of 1991, 115 cases of hepatitis A were reported in two heavily gay neighborhoods in New York's Manhattan. Only 17 cases were reported over the span there a year earlier, the CDC said.

In Denver, 24 cases of hepatitis A were reported among gay or bisexual men in the first half of last year, up from the typical incidence of zero to three cases.

And in San Francisco, the percentage of cases among gay or bisexual men rose by 10 percentage points in a year, although the numbers were higher to start with: 186, or 78 percent of male hepatitis A patients in the first 11 months of '91 identified themselves as gay or bisexual, compared with 64 of 94, or 68 percent, of male patients interviewed a year earlier.

The outbreaks ''indicate a substantial shift in the ... risk factors for hepatitis A in several cities throughout the world,'' the CDC said, adding that outbreaks in gay men occurred last year in Toronto, Montreal and the Melbourne and Sydney areas of Australia.

''Our concern is that the increase in hepatitis A in homosexual men may reflect a return to unsafe sex practices, or a misperception regarding the relative safety of certain sexual behaviors,'' said Dr. Miriam Alter, chief of hepatitis epidemiology for the CDC.

''Recently, the rates of gonorrhea and hepatitis B have sharply declined as a result of changes in homosexual behavior,'' she said. ''This increase may herald an increase in other sexually transmitted diseases.''

Hepatitis B, caused by a different virus, is transmitted through infected blood, typically by sexual contact or sharing drug needles. The nation reported some 23,000 cases of hepatitis A and 17,000 cases of hepatitis B last year.

There is no treatment for either form of viral hepatitis, which can cause fever, lethargy and pain. Some patients may be ill only briefly, but others can be ill for months; death or chronic complication is rare.