BACKGROUND: Drug use has been associated with risk behaviour among gay men. We examined the use of drugs and sexual risk behaviour among homosexually-active men who engaged in group sex in Australia. METHODS: We used an anonymous, self-complete survey about participants' most recent occasion of group sex with other men and in-depth interviews with a small number of these survey participants. The 746 men who reported having engaged in group sex within the previous 6 months were included in these analyses. RESULTS: Among 746 men who engaged in group sex within the previous 6 months, 63.0% reported using illicit drugs at the group sex encounter. Men commonly reported using drugs specifically to enhance their sexual experience and to intensify the pleasure of that experience. After controlling for each drug type and other risk factors, only use of methamphetamine (odds ratio = 1.74, confidence interval = 1.06-2.88, P = 0.030) and having more than five drinks (odds ratio = 2.41, confidence interval = 1.34-4.33, P = 0.003) were independently associated with unprotected anal intercourse with non-HIV seroconcordant partners in multivariate analysis. CONCLUSION: Methamphetamine and heavy alcohol use are associated with increased sexual risk behaviour among men who engage in group sex. Within more 'adventurous' gay community subcultures, drug use is often for the explicit purpose of enhancing the sexual experience and this complex relationship may be key to understanding HIV risk among these men.