To compare population-based prevalence estimates of sexual risk behaviours and HIV seroprevalence of male sex workers who have sex with men (MSM) and those not engaged in sex work in Campinas, Brazil MSM (n = 658) were recruited for a crosssectional study through respondent-driven sampling. Audio-assisted computer self-interview was used to collect information on sexual behaviours and HIV testing (optional) was performed. Population-based prevalence estimates with 95% CI of characteristics and behaviours of MSM sex workers and non-sex workers are reported. One-quarter reported ever receiving payment for sex and 14.8% (95% CI 11.1 to 19.0) had been paid in the previous 2 months; most exclusively with men. MSM sex workers were significantly more likely than non-sex workers to report being transgendered (40.5% vs 8.1%), to practise unprotected receptive (22.4% vs 4.6%) and insertive (20.5% vs 5.0%) anal intercourse with >2 male partners and to have unprotected vaginal sex with women (22.7% vs 5.6%). MSM sex workers experienced significantly greater rates of psychological abuse (80.9% vs 58.4%) and physical abuse (48.2% vs 15.2%): MSM sex workers have higher sexual risk behaviours as well as social vulnerabilities than the general population of MSM. HIV/sexually transmitted infection prevention efforts should be targeted to this riskier subgroup. Programmes should be transgender sensitive, should recognise that MSM sex workers have sex with men and women and address other factors that influence risk, such as homophobic abuse. Male sex workers who have sex with men (MSM) may have behaviours and vulnerabilities that increase their HIV risk. However, little is known about this population in Brazil. A cross-sectional survey was used to determine population-based estimates of sexual risk behaviours and HIV seroprevalence of MSM sex workers and non-sex workers.