Participation of MSM in group sex events (GSEs) is an understudied phenomenon. Studies on GSEs identified significant proportions of MSM engaging in unprotected anal intercourse (UAI). We sought to identify the prevalence of group sex participation among MSM in Washington, DC and to characterize these experiences. Data were collected for NHBS-MSM-3 in 2011. More than one-quarter of MSM (27.2 %) reported engaging in group sex in the prior year, with one-third reporting no condom use with their sex partners (33.0 %). In multivariable logistic regression, men who participated in a GSE in the prior year were significantly younger, more likely to be white, and to have used crystal meth, poppers, and downers in the past year. The high prevalence of UAI during GSEs, especially in view of the fact that HIV-positive MSM were significantly less likely to report condom use, offers an opportunity to develop risk reduction interventions specific to GSE attendees.