Background: Anonymous sex sites have long been the target of HIV prevention interventions. In the UK, the recent increase in venues offering sex on the premises has fuelled speculation about increases in HIV transmissions. However, there is little research regarding the role of these sites in the transmission of HIV. Methods: A self-completed questionnaire covering demographic characteristics, sexual behaviour and location of sexual activity was distributed during the London Gay Pride festival in July, 1997. The sample was male, UK resident and homosexually active in the last year (n=1695). Results: In the last year almost half (49.7%) had sex in anonymous sex sites including cruising grounds (31.1%); saunas (27.2%); pubs/ clubs (25.4%) and public toilets (20.6%). Sex in an anonymous site significantly varied by age, residence, education, and HIV testing history. It was most common among men in their 40s who were resident in London, with higher levels of education and who had tested HIV positive. Men who used anonymous sites had significantly more sexual partners than men who did not. Overall, 5.7% had unprotected anal intercourse (UAI) in an anonymous sex site in the last year (11.5% of those who used such a site). Having done UAI in an anonymous site was significantly more common with lower levels of education, and among men who had tested HIV positive. Discussion: Anonymous sex participants engage in more UAI than non participants but not disproportionately in anonymous sex sites. Therefore, engaging in increased UAI is a function of engaging in more sex rather then frequenting such sites. To target men with increased numbers of UAI partners, it is appropriate to do so in anonymous sex sites. However, a different type of intervention is appropriate for men who engage in UAI in anonymous sex sites. Campaigns that aim to improve risk reduction strategies will need to engage with the complex assumptions, beliefs and strategies informing anonymous UAI as well as factors particular to men with lower educational qualifications. Such an intervention is likely to present a very particular challenge to practitioners.