The finding, presented Monday during the first day of scientific sessions at the 14th International AIDS Conference, is a worrying sign that the epidemic could be in danger of accelerating again in the United States.
The study indicated that ignorance of infection among HIV positive gay and bisexual men was twice as common as previous estimates, which were based on HIV tests results of people entering the military or jobs that require screening.
Those have indicated that one in three people were unaware they were infected, said Phill Wilson of the African American AIDS Policy and Training Institute in Los Angeles, who was not connected with the research.
The survey, conducted by the U.S. Centers for Disease Control and Prevention, "helps explain, at least in part, why many young gay and bisexual men in the United States are becoming infected," said Duncan MacKellar, who led the study.
"Many gay and bisexual men have HIV, most do not test frequently and ... are engaging in behaviors that can transmit the virus," he added.
Researchers surveyed 5,719 men aged 15 to 29 at dance clubs, bars, health clubs and street locations in Baltimore, Dallas, Los Angeles, Miami, New York and Seattle from 1994 to 2000.
Of the 573 who tested HIV positive, 440 - or 77 percent - had not known they were infected with the virus. Around half of those men hadn't been tested in the past year and half were having anal sex without condoms, said MacKellar.
Ninety-one percent of the black HIV positive men didn't know they were infected, and 70 percent of the Hispanic HIV positive men were unaware of their status, MacKellar said.
Black men were the most likely to be infected and the least likely to know that they had the virus, he said.
On Sunday, Dr. Ronald Valdiserri, the CDC's deputy director for HIV, STD and TB prevention, warned that infection rates could climb again unless prevention efforts are stepped up. The rates have remained fairly stable at 10,000 new cases every three months since 1998.
Young gay men shun testing because of the stigma of being HIV positive, Wilson said.
The problem is worse for minorities because they are not being reached by normal awareness campaigns and are not accustomed to seeking preventive medical care.
"There are number of important messages that are not getting through to critical communities," he said, adding that "when black men see a doctor it's in the emergency room."
A weakness in the study, Wilson said, was that it lacked information on sexual partners of the surveyed men. Knowing the age of the partners or whether they belonged to high-risk groups would have helped identify patterns of infection, he said.
Another CDC study presented at the conference, which examined anonymous blood samples of 40,000 high-risk patients of all ages, found that the rate of new infections for gay and bisexual men was nine times higher than for women and heterosexual men.
Other CDC research linked psychosocial health problems such as depression and drug abuse to higher HIV infection rates and large age gaps within homosexual couples to a higher propensity to engage in risky sexual behavior.
By Jerome Socolovsky