Health in Men (HIM) was an open cohort study of 1,427 HIV-negative homosexual men in Sydney. The majority of respondents' unprotected anal intercourse (UAIC) events were with partners whose HIV status they did not know. Nonetheless, with casual partners with whom they engaged in UAIC, respondents indicated that they knew 'well' 28.9% of the HIV-negative partners and 26.2% of HIV-positive partners, but only 7.6% of the HIV status unknown partners. Respondents were more likely to have engaged in UAIC with partners they knew well (McNemar P < 0.001). The challenge for HIV prevention is that many gay men's decisions about condom use may be driven as much by their relationship with individual partners as their commitment to 'safe sex'.