Prime Time, a youth development intervention, aims to reduce multiple risk behaviors among adolescent girls seeking clinic services who are at high risk for pregnancy. The purpose of the current study was to examine whether Prime Time involvement produced changes in relational aggression, physical violence, and related psychosocial and behavioral outcomes. Qualitative case exemplars illustrated social contexts of intervention participants with differing longitudinal patterns of relational aggression and physical violence. Data were from a randomized efficacy trial with 13-17 year-old girls (n = 253) meeting specified risk criteria. Intervention participants were involved in Prime Time and usual clinic services for 18 months, control participants received usual clinic services. Participants in the current study completed self-report surveys at baseline and 18 months following enrollment. Outcomes analyses revealed significantly lower levels of relational aggression perpetration in the intervention group versus controls. In contrast, Prime Time involvement did not result in significant reductions in physical violence. Exploratory dose-response analyses indicated that reductions in relational aggression may have been most pronounced among girls actively involved in Prime Time case management and peer leadership activities. Qualitative findings suggested that the intervention's emphasis on modeling and building supportive relationships contributed to reductions in relational aggression. This study contributes to what has been a very limited evidence base regarding effective approaches to preventing violence among high-risk adolescent girls. Findings suggest that offering youth development interventions through clinic settings hold promise in reducing violence risk among vulnerable youth.