Unprotected anal intercourse with casual partners (UAIC) is the strongest predictor of HIV incidence among gay men. Familiarity between sex partners has been associated with likelihood to engage in UAIC, but the decision to use condoms with partners who are previously acquainted is complex and multifaceted. Using data from the Pleasure and Sexual Health survey 2009, we investigated the association between aspects of familiarity with casual partners and disclosure of HIV serostatus. Compared with occasions when they engaged in protected anal intercourse (PAIC), when men engaged in UAIC they were more likely to report having previously met their partners (PAIC 45.9%; UAIC 54.9%), knowing them very well (PAIC 7.9%; UAIC 19.7%), and having previously had sex with them (PAIC 32.2%; UAIC 44.8%) (McNemar P < 0.001). Men were also more likely to disclose their HIV serostatus to their casual partners on occasions of UAIC, were more confident they knew their partner's HIV serostatus and trusted them more. Overall, UAIC was associated with both the broad concept of 'familiarity' (composed of elements of prior acquaintance and trust) and HIV disclosure. When men engage in UAIC without some prior familiarity, disclosure of HIV serostatus, or confidence and trust in their partners, they are probably at greater risk than on occasions when they engage in UAIC with partners with whom they do have these qualities. However, for some men, their trust in knowing specific details about their partners may not always be well-informed or reliable. These different circumstances are challenging for HIV prevention work.