Background/Purpose:Juvenile‐onset Spondyloarthritis (JSpA), referred to as Enthesitis‐Related Arthritis (ERA) subtype under the International League of Associations for Rheumatology (ILAR) classification is characterized by arthritis and enthesitis largely affecting the lower limbs. Axial involvement is uncommon at presentation, but may develop in the second decade of life. Although there are validated instruments assessing spinal disease in adults with Ankylosing Spondylitis (AS) such as the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) and Bath Ankylosing Spondylitis Functional Index (BASFI), there are no validated tools to measure disease activity or functional impairment in this population of children. While we have previously reported excellent intra‐reliability of the BASDAI and BASFI, the aims of the current study were to measure the validity and responsiveness of these two adult scores in JSpA.Methods:Patients diagnosed with ERA (ILAR criteria) followed in the JSpA Clinic at The Hospital for Sick Children (June 2009–June 2010) were enrolled into the study. The BASDAI and BASFI were measured prospectively at baseline and again at 4 to 6 months. At each study visit, joint and entheseal clinical exams were performed. CHAQ and Physical Global of Disease Activity scores were recorded. The data collected at baseline and the follow up visit were used to assess construct validity and were expressed using Pearson's correlation coefficient. Responsiveness (sensitivity to change) was calculated in a subgroup of patients who showed changes in joint and entheseal counts over time by dividing the mean change between the two assessments by the standard deviation of the change scores and was expressed as the standardized response mean.Results:There were 38 patients (87% male) with a mean age at diagnosis of 12.1 ± 2.5 years and average age at enrollment of 14.5 ± 2.5 years. Average disease duration at the time of the study was 5.4 ± 1.8 years. 45% were HLA B27 positive with 18% had a family history of Spondyloarthritis. 71% had a history of clinical SI involvement. The average time between baseline and follow up clinic visits was 4.6 ± 2.3 months. Correlations between both the BASDAI and the BASFI and active joint counts were found to be high (r > 0.6) while correlations with sites of enthesitis were found to be low to moderate (r = 0.2 – 0.5). Responsiveness was greatest for the BASDAI and BASFI for detecting worsening arthritis (1.18 and 1.11, respectively). Correlations between the two instruments and CHAQ and Physical Global of Disease Activity scores were highly correlated at both time points.Conclusion:The current study demonstrates that the BASDAI and the BASFI show good construct validity and responsiveness and may be used in the evaluation of disease activity and functional impairment in children with JSpA. Correlations were higher for both measures in arthritis than in enthesitis, and sensitivity to changes over time was best for detecting worsening arthritis. The results of this study illustrate that these two instruments validated in adults may become an objective addition to developing Paediatric JSpA core sets.