Objective. To determine the prevalence of temporomandibular joint (TMJ) disease in a cohort of children with new-onset juvenile idiopathic arthritis (JIA), and to compare magnetic resonance imaging (MRI) with ultrasound (US) for the detection of acute and chronic changes of TMJ arthritis. Methods. Between January 2005 and April 2007, children with newly diagnosed JIA were prospectively evaluated for TMJ arthritis. Prior to imaging, jaw pain and disability were assessed with questionnaires and physical examination. The TMJs of all patients were imaged with MRI and US within 8 weeks of diagnosis. Results. Of the 32 patients enrolled, 78% were female, and the median age was 8.6 years (range 1.5-17.2 years). Acute TMJ arthritis was diagnosed in 75% of the children by MRI and in none by US; chronic arthritis was diagnosed in 69% by MRI and in 28% by US. Findings of both acute and chronic TMJ disease were detected by MRI in 53% of the patients. Of those with acute TMJ arthritis, 71% were asymptomatic, and 63% had normal findings on jaw examination. Fifty-six percent of patients with acute disease had an improved maximal incisai opening after corticosteroid injection. Among these responders, 56% had been asymptomatic and had normal jaw examination findings. Conclusion. TMJ arthritis was present in the majority of patients with new-onset JIA. Findings on MRI along with responses to treatment among asymptomatic patients with normal jaw examination findings suggest that a history review and physical examination are not sufficient to screen for TMJ disease. Our results also suggest that MRI and US findings are not well correlated, and that MRI is preferable for the detection of TMJ disease in new-onset JIA. © 2008, American College of Rheumatology.