Background:  Propagation of a sexual HIV epidemic requires that HIV⁺ persons have unprotected sex with HIV^- persons at levels sufficient to sustain transmission. “HIV serosorting,” where HIV^- persons preferentially select other HIV^- persons as sexual partners and HIV⁺ persons select HIV⁺ partners, is one strategy that could reduce HIV transmission on an individual and community level. We examined trends in partner serostatus among men who have sex with men (MSM) seeking anonymous HIV testing in San Francisco.

Methods:  Data on sexual risk behavior and HIV seroconversion were obtained from all MSM seeking anonymous HIV testing in San Francisco from 1998 to 2002 (n = 15,059). Persons seeking HIV testing were assumed to consider themselves as HIV^- prior to obtaining test results. Analysis used the chi-square test for trend.

Results:  The proportion of MSM testers reporting sex with a known HIV⁺ partner within the previous 12 months declined steadily from 1998 to 2002 (p <0.0001). There was a concurrent decrease in reported unprotected oral sex (p <0.0001) and unprotected anal sex (p = 0.023) with a known HIV⁺ partner during the same period.

Conclusions:  Anonymous testing data in San Francisco may indicate that HIV^- MSM are increasingly selecting partners known or presumed to be HIV^-, suggesting an increase in HIV serosorting. Moreover, the decrease in unprotected oral sex and unprotected anal sex with a known HIV⁺ partner suggests that condoms are being used increasingly in serodiscordant partnerships. Our findings provide cautious optimism that continuing rises in overall unprotected sex and sexually transmitted diseases observed in recent years among San Francisco’s MSM population may not necessarily translate into concurrent increases in HIV incidence.

Keywords: men who have sex with men (MSM); HIV serosorting; sexual risk behavior