© 2014, American College of Rheumatology. Results. In total, 106 physicians representing 14 countries completed the initial questionnaire. Completion rates for the subsequent questionnaires were 84%, 75%, and 77% of the original respondents. Ten items exceeded 80% consensus: arthritis, enthesitis, patient pain assessment, inflammatory markers, morning stiffness, clinical sacroiliitis, uveitis, back mobility, and patient and physician assessments of disease activity. After item analysis, 2 items were eliminated (patient and physician assessments of disease activity). Factor analysis identified 3 primary domains that explained 58% of the variance: peripheral disease, axial disease, and uveitis. The Cronbach's α coefficient was 0.66. The JSpADA had high or moderate correlations with the Juvenile Arthritis Disease Activity Score (r = 0.81), patient and physician assessments of disease activity (r = 0.70 and r = 0.66, respectively), and the Childhood Health Assessment Questionnaire (r = 0.56). The JSpADA discriminated well between subjects with active versus inactive disease (P < 0.001) and was responsive to improvement or worsening in disease activity over time (P < 0.001).Conclusion. Using international input and consensus formation techniques, we developed and validated the first disease activity assessment for juvenile spondyloarthritis. Future studies should validate the JSpADA in a prospective multicenter cohort.Objective. To develop and validate the Juvenile Spondyloarthritis Disease Activity Index (JSpADA) for use in clinical practice and research.Methods. Using modified Delphi consensus techniques, 10 items were selected by participants in the international pediatric rheumatology listserv, the Childhood Arthritis and Rheumatology Research Alliance, and the listserv for the pediatric section of the American College of Rheumatology. Validation was performed in a retrospective multicenter cohort of 244 children.