The aims of this study were to understand strategies and consistency of strategy used by HIV-negative and HIV-positive men who have sex with men (MSM) to ascertain the HIV status of their male sexual partners and their associations with unprotected anal intercourse (UAI) and serodiscordant UAI (SDUAI) in the past 3 months. Participants (n = 640) completed an online survey in December 2007. The most commonly reported strategy was checking online profiles (85%), followed by talking before sex (82%), talking after sex (42%), and guessing (29%). Adjusting for demographic and behavioral factors, guessing the HIV status of sex partners was associated with greater UAI and SDUAI partners, as was using an inconsistent strategy. Ascertaining HIV status before having sex was associated with fewer SDUAI partners. Prevention should target MSM who guess the HIV status of their sex partners and emphasize explicit safer sex agreements.