This study aims to better understand STI/HIV-related vulnerability among self-identified lesbians and bisexual young women aged 18-26 years in two neighbourhoods in Rio de Janeiro. Based on ethnographic observations and in-depth interviews, the paper analyses their life experiences and trajectories. Findings reveal that sexual identities and ties attributed to sexual interactions with women and men have an important role in influencing perceptions of vulnerability. The notion of STI and HIV risk is not well developed among women who have sex with women. It emerges largely in practices with bisexual female partners and those of the opposite sex, since in these the potential for HIV transmission (through contact with semen) is recognized. Sexually transmitted infection and HIV-related risk with male partners is seen as small by the women in the study, since such relationships are seen as 'occasional' and generally speaking occur with someone they know well. The value given to trust and the lack of alignment between sexual identities and sexual practices reinforces the need for approaches to STI and HIV prevention and care that prioritise the sexual history and practices of women and their sexual identities within specific contexts.