Background:  Serosorting refers to the practice of preferentially choosing sex partners, or deciding not to use condoms with selected partners, based on their concordant HIV status.  

Methods:  Published data as well as unpublished data collected through the PHSKC STD control program will be summarized.

Results:  Men who have sex with men (MSM) have engaged in serosorting since at least the early 1990s, though data are conflicting on whether the practice is increasing. Serosorting protects HIV negative men from HIV, with MSM who report unprotected anal intercourse only with HIV negative partners experiencing a significantly lower risk of HIV acquisition than men who report unprotected anal intercourse with HIV positive partners or partners whose HIV status is unknown. However, in King County, Washington, as many as half of persons with newly diagnosed HIV deny nonconcordant unprotected anal intercourse, suggesting that serosorting often fails. The risk of bacterial sexually transmitted diseases in MSM who serosort is comparable to that of MSM who do not, and is significantly higher than that observed among men who deny all unprotected anal intercourse. At a population level, the prevention implications of serosorting are complex, substantially undefined, and depend on the extent to which the practice is an alternative to safer or less safe sexual behaviors. 

Conclusions:  Serosorting is common and provides substantial, but imperfect, protection against HIV. Efforts to increase serosorting, particularly if coupled with efforts to increase the frequency of HIV testing, would probably decrease HIV transmission as long as they were not associated with substantial increases in overall unprotected sex.