Among 746 participants in the Three or More Study (TOMS) of gay men who engaged in group sex in the previous 6 months, 22.4% reported unprotected anal intercourse (UAI) with any partners they did not know to be the same HIV serostatus as themselves. Not knowing oneself to be HIV-negative, not having a clear intention to use condoms, and more frequent group sex were independently associated with UAI. This study shows that gay men who engage in group sex represent an important priority for targeted HIV prevention activities and research.