This paper addresses the question of whether HIV-negative gay men engage in "serosorting" in casual encounters. Serosorting, defined as engaging in unprotected anal intercourse with casual partners who they report to be HIV negative, has been increasing among HIV-negative gay men in Sydney. Prevention and intervention programmes are urgently needed to alert HIV-negative gay men to the risks associated with "serosorting", and remind them of the need for consistent condom use.