A probability sample of 206 men who have sex with men from 16 sites in Phnom Penh were surveyed about sexual behaviors and tested for HIV and sexually transmitted infections (STIs). HIV and syphilis prevalence was 14.4% and 5.5%, respectively. Out of the total sample, 81% reported anal sex with any male partners in the past 6 months, and 61.2% reported having had vaginal sex. In the past 6 months, 82.8% of the sample reported having male partners who paid them to have sex. Self-reported sexual orientation did not match well with self-reported sexual behavior. Significant risk factors for HIV infection were anal sex with multiple partners, unprotected vaginal sex with commercial female partners in the past month, and any STI. Complex sexual networks indicate that men who have sex with men act as a bridge between higher and lower HIV prevalence populations. Better prevention efforts structured around behaviors rather than sexual identities are needed.