Background Evaluation of the level of disease activity is a fundamental component of the clinical assessment of children with JDM. The global tools that are currently available for the assessment of the overall disease activity in JDM are centered on physician's evaluation, neglecting parent's or child's perception. Furthermore, these instruments are lengthy and complex. There remains the need for a concise and easily administered tool that provides an absolute measure of disease activity for use in future trials in JDM Objectives To develop and test a new composite disease activity score for JDM, named the Juvenile Dermatomyositis Activity Index (JDMAI). Methods The JDMAI includes 4 measures: 1) physician global assessment of disease activity on a 0-10 VAS, 2) parent/patient global assessment of well-being on a 0-10 VAS, 3) muscle strength assessment, and 4) cutaneous disease activity. Validation analyses were conducted on 140 patients included in a multinational study and were based on evaluation of construct validity, responsiveness to change, and discriminant validity. Four versions of the JDMAI were tested, which differed items 3) and 4). Three versions included the hybrid MMT/CMAS (hMC), reversed and divided by 10, as measure of muscle strength (range 0-10), and the cutaneous domain of the DAS (range 0-9) (JDMAI-1), a cutaneous VAS (range 0-10) (JDMAI-2), or the skin involvement type and distribution items of the DAS (range 0-7) (JDMAI-3) as measures of skin activity. A fourth version of the score (JDMAI-4) included the 3 CMAS items of the hMC (head lift; sits up, floor rise) (range 0-20) for muscle strength and the skin involvement and distribution items of the DAS for skin activity. Results Construct validity: Spearman's correlations of all JDMAI versions were: strong (r >0.7) with total DAS (0.80 - 0.90), parents' disease activity VAS (0.73 to 0.80), and CHAQ (0.72 to 0.80); moderate (r 0.4-0.7) with CMAS (-0.63 to -0.65, -0.80 for JDMAI-4), pain VAS (0.55 to 0.60), fatigue VAS (0.62 to 0.70); poor (r <0.4) with LDH (0.27 to 0.32), and ESR (0.35 to 0.38). Correlation of all JDMAI versions with the Myositis Damage Index and CPK was not significant. Responsiveness to change between 2 consecutive visits: SRM ranged from 0.72 (JDMAI-1) to 0.78 (JDMAI-4). Discriminant validity: all JDMAI versions discriminated between patients rated in remission, continued active disease, and flare by the physician (p<0.001) and by the parent (p<0.001), and between patients with high, moderate, or low disease activity according to the physician (p<0.001). Conclusions All JDMAI versions showed good construct validity and responsiveness to change, and excellent discriminant validity. We have shown that the JDMAI is a valid instrument for the assessment of disease activity in JDM and is, therefore, potentially applicable in standard clinical care, observational studies, and clinical trials. Disclosure of Interest None declared