Young people constitute a priority for sexual health research, policy and planning. Many studies, however, regard youth as a homogeneous group defined by developmental stages and their problems as inherent rather than factors resulting from structural vulnerability. Ethnographic data from this study provided strong evidence of the inappropriateness, in prevention interventions, of the concept of 'young people' as a group defined only by age and gender. When incorporating social resources and support into the analysis, specific segments of youth with diverse sexual practices and health seeking behaviours emerge. Thus, although most young people in urban areas show a similar level of HIV/STI knowledge, their exposure to risk varies according to their living conditions. Two population segments - "street guys" and "fast girls" - identified as vulnerable for sexual risk, are characterized. Both groups hang out on the streets, and most are involved in using alcohol and drugs, and/or practicing transactional sex. This study provided evidence for the need of various approaches according to level of poverty and social vulnerability in order to develop more effective HIV/AIDS and STI prevention programs to meet the needs of young men and women in low-income areas. © 2005 Taylor & Francis Group Ltd.