It can be difficult to conduct research in pediatric rheumatology due to low disease prevalence and rheumatologist workforce shortages. The Childhood Arthritis and Rheumatology Research Alliance (CARRA) Registry aims to address the challenges associated with pediatric rheumatology research by collecting longitudinal data from patients across multiple centers. However, examples of other disease registries suggest there can be systematic differences between patients who choose to enroll, compared to those who choose not to and those who are lost to follow-up. Therefore, our goal is to assess whether to what extent this may be the case in the CARRA Registry.