This study represents the first systematic attempt to examine a theory-based program designed to reduce girls' social aggression and increase positive leadership among peers. Fifth-grade girls from six public schools were randomly assigned within classrooms to the social aggression prevention program (SAPP) and the comparison reading clubs. A school-based small group program, the SAPP was demonstrated to have a positive impact for all students in the domain of social problem solving. For students with high baseline social problems, teachers reported positive changes in SAPP participants' prosocial behavior. In addition, the content of the program was critical: reading club participants improved their reading achievement at greater rates than SAPP participants. Targeting female students in a program focused on resolving social conflicts from multiple perspectives may be an important addition to broader, multilevel initiatives to prevent aggression and promote leadership in schools.