Nearly 20% of gay and bisexual men in 21 large U.S. cities are HIV positive, but more than 40% do not know their status, according to study findings described in the September 24, 2010, Morbidity and Mortality Weekly Report.

Investigators from the CDC collected data from the National HIV Behavioral Surveillance system on HIV prevalence among men who have sex with men (MSM), defined as having had sex with another man at least once during the past year. The analysis included more than 8,000 men in 21 metropolitan statistical areas surveyed during 2008. Participants completed anonymous interviews and received HIV blood tests.

Overall HIV prevalence (total infections) was 19%. Rates were highly variable across cities, however, ranging from 6% in Atlanta to 38% in Baltimore. Early epicenters of the epidemic had above-average to high rates, including Chicago (19%), Los Angeles (19%), San Francisco (23%), and New York City (29%). Several higher-prevalence cities were located in the south, including Dallas (26%), Houston (26%), Miami (25%), and New Orleans (21%). But Washington, D.C. -- often cited as one of the most heavily impacted cities -- was below average at 14%.

Looking at demographic factors, men who identified as homosexual and bisexual had similar HIV prevalence rates (19% and 18%, respectively), more than twice as high as self-identified heterosexuals (8%). Prevalence rose with increasing age -- from 7% among men age 18-19 up to 28% for the 40-49 age group and 25% for those age 50 or older.

Consistent with other studies, African Americans had the highest prevalence (28%), Asians had the lowest (8%), and whites and Hispanics were in between (16% and 18%, respectively). Prevalence fell as education level and income increased.

Overall, 44% of the men who tested HIV positive as part of this study had not previously known they were infected. By self-identification, 39% of homosexual men knew their status compared with 63% of men who said they were bisexual or heterosexual. Racial/ethnic disparities were again apparent, with 59% of black men, 46% of Hispanics, 43% of Asians, and 26% of white men not knowing they were positive. Lack of awareness was especially high among young men, reaching 75% for the 18-19 age group.

"Because MSM represent the only group with increasing HIV incidence and comprise the largest proportion of new infections, it is critical to target resources and prevention strategies to MSM," the study authors wrote.

Liz Highleyman ( is a freelance medical writer based in San Francisco.