Background: Although exposure and response prevention (ERP) is an effective treatment for youth with obsessive–compulsive disorder (OCD), the majority of studies, randomized clinical trials of individual therapy, find variability in treatment response. We evaluated the potential role of individual differences in OCD presentation, comorbid disorders, age, and gender on treatment effects. Moreover, we examined these potential effects in a group format in a naturalistic, clinic-based sample of patients. Methods: Pediatric patients with a DSM-IV diagnosis of OCD (n=41) were treated with ERP in an intensive outpatient community-based program. OCD, mood, and anxiety symptom severity was measured at baseline, during treatment, and at discharge. Trajectories and predictors of treatment outcome were measured using linear growth models. Results: We found that group-based ERP was effective in reducing pediatric OCD symptom severity in a naturalistic treatment setting irrespective of age or gender. Furthermore, ERP was found to be effective at reducing depressive symptoms but not other anxiety symptoms. We also found inter-individual variability in the discharge levels of contamination, symmetry, and intrusive sexual thoughts and in the rate of severity reduction of intrusive sexual thoughts. Conclusion: Group-based ERP is an effective treatment for children and adolescents with OCD. Several factors, including symptom dimensions and comorbid psychopathology, are associated with treatment response and outcome in this pediatric population. Depression and Anxiety, 2011. © 2011 Wiley-Liss, Inc.