The present study examined the effect of Obsessive Compulsive Disorder (OCD) on child and adolescent sibling relationships. In previous research on adult OCD, siblings of OCD sufferers showed a high level of occommodotion to obsessive-compulsive symptoms and subsequent depression. No research has been conducted on the effect of OCD on child and adolescent sibling relationships. This small but important pilot study investigated sibling accommodation to obsessive-compulsive symptoms, sibling relationship quality, and depression and anxiety in siblings of children who have OCD. The study had two components. First, siblings of children with OCD were examined before and after a 14-week treatment program for OCD. Second, these siblings were compared at prebefore treatment with a comparison group of siblings whose brother or sister did not meet diagnostic criteria for either OCD or any other childhood disorders (i.e., comparison group). Three outcomes were noted. Siblings of OCD children accommodated to obsessive-compulsive symptoms and were distressed by the presence of OCD in their brother or sister. Furthermore, the quality of sibling relationships in OCD families improved with successful treatment of OCD. Finally, anxiety and depression were higher in siblings of an OCD child or adolescent, compared to siblings in the comparison group. Given the significant impact that OCD has on the sibling relationship, implications for family-based treatment, incorporating a sibling support component, are addressed.