The Children's Yale-Brown Obsessive Compulsive Scale (CY-BOCS) is the instrument of choice for assessing symptom severity in older children (i.e., 8-18 years) diagnosed with obsessive-compulsive disorder (OCD). The reliability and validity of this measure for use among younger children (i.e., 5-8 years of age), however, has never been examined. The primary aim of this study was to examine this scale's use among those presenting with early childhood OCD. Forty-two children with OCD between the ages of 4 and 8 years of age were recruited as part of a larger treatment outcome study, and the reliability and validity of the CY-BOCS was examined. Results revealed questionable reliability for the measure's 5-item Obsessions subscale but good reliability (i.e., internal consistency, temporal stability) for the 5-item Compulsions subscale and 10-item total scale. Results also revealed that the CY-BOCS total scale demonstrated mixed discriminant validity but strong convergent validity and sensitive to change. Collectively, the 10-item, CY-BOCS total score yields a reliable and valid scale for the assessment of symptom severity in early childhood OCD. However, we urge caution in use of the Obsessions subscale in isolation for either clinical or research purposes. Limitations and future areas of research are discussed including the potential benefit of developing a measure of OCD-related symptom severity specifically for younger children with greater attention to developmental differences among children within this population.