This study was designed to determine the effectiveness of group activity therapy as a school based intervention with fourth and fifth grade preadolescents with learning disabilities experiencing behavior problems. The group activity therapy intervention followed humanistic principles and was designed to address the cognitive and social emotional needs of this population. The preadolescents were provided a variety of developmental appropriate materials and activities to encourage self expression and group interaction. The 24 volunteer preadolescents were randomly assigned to the experimental group (n=12) and to the control group (n=12). The treatment group preadolescents were divided into groups of three and participated in group activity therapy one hour per week for 12 weeks. The participants were assigned to groups according to individual needs and personality traits. The control group received no treatment during the study. Pre and post test data were collected from parents using the Child Behavior Checklist (CBC) and the Behavior Assessment Scale for Children (BASC). Analysis of Covariate (ANCOVA) was utilized to determine statistical significance between the treatment group and the control group on the post-test means for each hypothesis. In each case, the post-test specified in each hypothesis was used as the dependent variable and the pre-test as the covariate. Specifically, the preadolescents in the treatment group showed statistically significant decreases in total behavior problems on the BASC (p=.05) and decreases in internalizing problems on both the BASC and CBC (p=.03, p=.05, respectively). While not statistically significant, positive trends were noted on the CBC total behavior scale (p=.08) and on the CBC externalizing scale (p=.09). In addition, Cohen's d effect size was calculated for each hypothesis and post hoc analysis of the subscales to determine practical significance of the treatment on the experimental group when compared to the control group. A large treatment effect size was found on the BASC (d=.91) and CBC (d=.82) total behavior problems scales and on the BASC (d=1.03) and CBC (d=.90) internalizing problems scales. A moderate to large treatment effect size (d=.78) was found on the CBC externalizing problems scale and a medium treatment effect size (d=.53) was found on the BASC externalizing problems scale. Qualitative data was also examined to determine clinical significance of the intervention. This study determined that group activity therapy is an effective intervention for preadolescents diagnosed with a learning disability.