OBJECTIVE: Enthesitis is a major feature of juvenile idiopathic arthritis (JIA) but is difficult to diagnose clinically. Our objective was to compare the accuracy of ultrasonography with power Doppler (US-PD) versus clinical examination for diagnosing enthesitis in patients with JIA and healthy controls. METHODS: Twenty-six consecutive patients with JIA and 41 healthy volunteers underwent standardized clinical and US-PD examinations of 5 entheseal sites (proximal and distal quadricepital tendon insertions, Achilles tendon, and plantar fascia). US-PD reproducibility was evaluated. US-PD enthesitis was defined as a PD signal at the enthesis insertion. Bursitis, erosions, and cartilage vascularization were recorded. RESULTS: In the JIA group, 27 (12.5%) of the entheseal sites exhibited clinical enthesitis (distal patellar ligament in 45% of cases) and 20 (9.4%) exhibited US-PD enthesitis (distal patellar tendon in 30%), including 10 clinically normal sites (50%). US-PD enthesitis was found in several patients with oligoarthritis or polyarthritis. Clinical enthesitis (P < 0.0001) and HLA-B27-positive (P = 0.05) status were significantly associated with US-PD enthesitis. Erosion and bursitis, but not tendon thickening, were associated with US-PD enthesitis. US-PD enthesitis was not found at any of the 410 entheseal sites in controls; grade 1 cartilage vascularization was noted at 6% of the control sites. CONCLUSION: Enthesitis is a rare phenomenon in JIA. Clinically silent enthesitis is detected by US-PD and can be found in JIA categories other than enthesitis-related arthritis. Tendon thickening and cartilage vascularization can be detected in healthy controls. These findings may have implications for patient classification of the use of US-PD.