Background:The aim of this work was to study the ultrasonographic (USG) features of knee joints in relation to clinical and laboratory measures in patients with juvenile rheumatoid arthritis (JRA), and also to evaluate the accuracy of ultrasound in the diagnosis of local joint activity.Methods:This study included 20 with JRA and 20 matched and apparently healthy controls. All patients were subjected to full history taking, careful clinical examination and laboratory investigation. The knee joints of all patients and control were examined with plain radiography and ultrasonography on the same day of clinical examination using ultrasound to detect synovial thickness and effusion at the knee.Results:Mean USG knee synovial thickness was significantly greater in JRA patients versus controls (4.2 ± 2.4 mm versus 1.7 ± 0.3 mm, P < 0.001). Although knee effusion was not detected in any of the controls, it was demonstrated in 90% of JRA patients, with a mean effusion volume of 3.8 ± 3.1 mL. There was a statistically significant difference (P < 0.001) between clinically active and inactive knees with regard to knee synovial thickness. Mean knee effusion volume was significantly (P < 0.05) higher in the clinically active than in the clinically inactive knees. Patients with high disease activity had a significantly (P < 0.05) higher knee synovial thickness and knee effusion volume than patients with low and moderate disease activity. Significantly (P < 0.05) positive correlations were found between knee synovial thickness and articular index (AI) scores, disease activity score, clinical knee scores, erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP) levels. Significant positive correlations (P < 0.05) were found between knee effusion volume and AI scores, visual analog scores, disease activity scores, clinical knee scores, ESR, and CRP levels. Significant negative correlations (P < 0.05) were found between knee effusion volumes and hemoglobin levels.Conclusion:UGS-detected parameters represent a reliable index of JRA disease activity with a higher sensitivity for knee synovial thickness and higher specificity for knee effusion.