1. Ellen A. Goldmuntz, MD, PhD* 2. Patience H. White, MD* 1. *Children’s National Medical Center, Department of Pediatrics, George Washington University School of Medicine and Health Sciences, Washington, DC Juvenile idiopathic arthritis (JIA) is the most common rheumatic disease of childhood. It also is one of the more common chronic diseases of childhood, occurring as frequently as juvenile diabetes mellitus, four times more frequently than cystic fibrosis and sickle cell anemia, and 10 times more frequently than acute lymphoblastic leukemia, hemophilia, or muscular dystrophy. JIA is a new classification of juvenile arthritis developed by the International League Against Rheumatism (ILAR) that is used worldwide; it is replacing the previous American classification of juvenile rheumatoid arthritis (JRA). The cause of JIA is unknown, but substantial evidence suggests that the pathogenesis is autoimmune. The diagnosis of JIA is one of exclusion and is made by using a combination of historical, clinical, and laboratory data. The disease course is highly variable, and in the past, JIA had been viewed as a more benign illness than is recognized currently. Recent data reveal that a substantial number of children diagnosed with JIA have active disease that persists into adulthood and results in functional limitation. Thus, the management of JIA has become more aggressive earlier in the course of disease to limit permanent disability. JIA is defined as the presence of objective signs of arthritis in at least one joint for more than 6 weeks in a child younger than age 16 years after other types of childhood arthritis have been excluded. Arthritis is defined as the presence of swelling of the joint or two or more of the following: limitation of motion, tenderness, pain with motion, or joint warmth. Many causes of arthritis must be excluded before JIA can be diagnosed definitively (Table 1). These causes include, but are not limited to, infectious and postinfectious etiologies (septic arthritis, acute rheumatic fever, Lyme disease), hematologic and neoplastic disease, connective tissue diseases (systemic lupus erythematosus, juvenile …