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).