Prevalence of unprotected anal intercourse between casual male partners (UAIC) has been increasing worldwide. We explored trends in serodiscordant UAIC and the associated factors among gay men in Sydney. Proportions of HIV-positive and negative men with serodiscordant casual partners increased during 2003-2006. Prevalence of serodiscordant UAIC increased among HIV-negative men. Age, number of partners, seeking partners online, drug use and esoteric practices were associated with serodiscordant UAIC. Increases in serodiscordant UAIC may be related to growing disclosure. These findings do not indicate a core group of high-risk men. More research is needed about the context in which serodiscordant UAIC happens.