Relational aggression has quickly become a serious issue in schools. In response, school professionals have sought and developed interventions despite a dearth of empirical examination and support. The current study bolsters this area by examining the initial efficacy of GIRLSS, an intervention developed over multiple iterations incorporating the feedback and perspective of involved schools. GIRLSS is a 10-week school-based group counseling, parent training and parent phone consultation intervention based on cognitive behavioral strategies and social learning theory. The current study used a randomized, pretest-posttest design with assignment to either a GIRLSS intervention group (N = 22) or a waiting list control group (N = 12). Results of multiple regression analyses found that participation in the intervention group significantly predicted reduced levels of school counselor-reported relational aggression reported. Participation in the intervention group also significantly predicted increased knowledge of relational aggression and the GIRLSS curriculum. Increased understanding mediated the relationship between teachers' pretest and posttest report of participants' relationally aggressive behaviors. However, no significant effects were found on proposed secondary outcomes or participants' self-report of RA. Limitations of these findings are discussed, including a small sample size, novel referral and recruitment procedures, and the validity of some outcome measures. Future research should seek to improve GIRLSS in collaboration with interested schools and utilize more sophisticated, feasible and valid recruitment and measurement procedures. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]