Group sex is relatively common among gay men, and is a likely risk factor for infection with HIV and other sexually transmissible infections (STI). The 5,432 participants in the 2008 Gay Community Periodic Survey in Sydney, Melbourne and Brisbane, Australia, were asked about their participation in group sex. Nearly half (44.0%) of those who reported sex with casual male partners in the previous 6 months also reported engaging in group sex. Among other factors, using drugs odds ratios (OR) and confidence intervals (CI) (OR = 1.89, CI = 1.61-2.21), being HIV-positive (OR = 1.62, CI = 1.13-2.32), and engaging in anal intercourse, both with a condom (OR = 3.03, CI = 2.46-3.73) and without a condom (OR = 5.68, CI = 4.53-7.12) were independently associated with having engaged in group sex. This study suggests that gay men who engage in group sex represent an important priority for targeted HIV and STI prevention activities and research.