Role-based sexual identities structure male same-sex partnerships and influence HIV/STI epidemiology among MSM in Latin America. We explored shifting relationships between sexual roles, identities and practices among MSM in Lima, Peru, and implications for HIV/STI prevention. Patterns of HIV/STI epidemiology reflected differential risks for transmission within role-based partnerships with relatively low prevalences of HIV, syphilis, and HSV-2 but higher prevalences of urethral gonorrhea/chlamydia among activo MSM compared with moderno and pasivo participants. Qualitative analysis of how MSM in Peru integrate sexual identities, roles, and practices identified four key themes: pasivo role as a gay approximation of cultural femininity; activo role as a heterosexual consolidation of masculinity; moderno role as a masculine reconceptualization of gay identity; and role-based identities as social determinants of partnership, network, and community formation. The concept of role-based sexual identities provides a framework for HIV prevention for Latin American MSM that integrates sexual identities, practices, partnerships, and networks. © 2013 Springer Science+Business Media, LLC.