Objectives: To determine prevalence of and risk factors associated with HIV and syphilis seropositivity and estimate incidence of HIV infection among Peruvian men who have sex with men (MSM) and characterize behaviors of men who report sex with both men and women ('bridgers'). Design: Cross-sectional study of MSM in Lima, Peru. Methods: Four-hundred and fifty-one MSM (of whom 442 responded to the question regarding sexual orientation) recruited through street outreach. Each was interviewed and underwent serologic testing for syphilis and HIV, including the less sensitive enzyme immunoassay test to estimate HIV incidence. Results: Overall, HIV and syphilis prevalence were 18.5% and 16.0%, respectively, with highest prevalence among cross-dressers (33.3% and 51.1 %, respectively). The estimated overall HIV seroincidence was 11.2% per year (95% confidence interval, 4.8-23.6). Overall, 47.1% of men reported ever having sex with a woman: 78.6% of men self-identifying as heterosexuals, 85.1% of bisexuals, 35.5% of homosexuals, and 12.5% of cross-dressers. Of these, 26.5% were 'bridgers', of whom 55% reported two or more female partners during the last year. 'Bridgers' were less likely to have always used condoms during the past year for vaginal sex (17%) than for insertive anal sex with men (25.5%). Conclusions: Among MSM in Peru, HIV and syphilis prevalence and HIV incidence were high, especially among cross-dressers. The high prevalence of bisexuality and low rates of consistent condom use, especially with female sexual partners indicates potential HIV transmission into the heterosexual population. © 2002 Lippincott Williams & Wilkins.