Objective. To examine the role of ultrasound versus magnetic resonance imaging (MRI) in assessing joint inflammation in patients with juvenile idiopathic arthritis (JIA) of the knee. Methods. This study was conducted on 38 patients with juvenile idiopathic arthritis (25 girls and 13 boys), whose ages ranged between 2-17 years (mean 8 years), presenting with joint swelling, tenderness, pain on motion and/or limitation of movement. Plain radiography, high-resolution ultrasound and MRI examinations of the knee (before and after contrast administration) were made on all patients. A control group of ten subjects was also examined. Results. Compared to the control group, sonographic examination was found to be of great value as regards the joint effusion, popliteal cysts, lymph nodes and to a lesser extent, the degree of affection of the articular cartilage. MRI was superior in evaluating the extent of synovial proliferation (pannus), thinning out and erosions of articular cartilage, loculated effusions as well as hypoplastic menisci and ligaments, especially after contrast enhancement. Conclusion. Ultrasound is a simple, inexpensive and valuable tool in evaluating the initial stages of JIA. In more advanced stages of JIA and also for monitoring the progression of the disease process and response to therapy, MRI examination following gadolinium proved to be superior in evaluation of the joint affection.