Objective: The aim of this study is to evaluate capability of ultrasound to assess disease activity in juvenile idiopathic arthritis compared to clinical and laboratory evaluation.Methods: Forty patients underwent clinical evaluation of 28 joints. Joints were assessed for swelling and tenderness. The same joints were scanned for synovial hyperplasia, joint effusion, and power Doppler (PD) signal.Results: In total, 1120 joints were assessed both clinically and with US. On clinical examination, 182 joints (16.3 %) were swollen, 139 joints (12.4%) were tender. On US evaluation, 192 joints (17.1% of total ) had synovial hyperplasia 39 joints (3.5% of total) had joint effusion and 142 joints (12.7 % of total number of examined joints) had power Doppler (PD) signal.A total of 196 (17.5%) and 210 (18.8%) joints had clinical and US synovitis, respectively. Of the 924 clinically normal joints, 32 ( 3.5% of clinically normal joints) had subclinical synovitis (i.e., had synovitis on US only). Ultrasound hyperplasia was in highest correlation with clinical measurement of joint swelling, while power doppler ultrasound was in highest correlation with measurement of joints tenderness and joint effusion has the least correlation with both.Conclusion: Study results showed ultrasound to be a reliable method in detection of synovitis and assessment of disease activity in juvenile idiopathic arthritis.