This multicenter, international longitudinal study will recruit JIA patients (according to ILAR classification criteria) with active disease according to JADAS 10 and 27 scoring prior starting recommended treatment. All JIA relevant data (demographics, duration, disease activity, medication usage and treatment efficacy) will be collected and parent/guardian written consent obtained. At enrolment and during predefined scheduled follow up visits (at 3 months up to 12 months) all JIA patients will be clinically evaluated by JADAS 10 and 27 scoring, examined by ultrasound gray-scale (GS) and Power Doppler (PD) in (44 joints) using OMERACT synovitis scoring system by an expert in pediatric ultrasound. At each visit blood samples will be obtained for evaluation of inflammatory markers (such as cytokines, chemokines and S100A8, S100A9 and S100A12). In the case of disease worsening, the same parameters will be performed as unscheduled visit.