The cause of juvenile rheumatoid arthritis (JRA) is not well-understood. Most experts believe it is caused by a combination of factors, including:

• An overly active immune system that inappropriately attacks joint tissues, as though they were a foreign substance.
• Viral or bacterial infections, which are a suspected trigger of the autoimmune process.
• Genetic factors that make a child's immune system more likely to react inappropriately. A study of relatives of children with JRA reported a higher occurrence of other autoimmune diseases in these families. It is possible that these families share genes that make them more susceptible to autoimmune diseases, including JRA.¹

An increasing number of international experts are now referring to JRA as juvenile idiopathic arthritis (JIA); idiopathic means "of unknown cause." As the international terminology becomes more widely used, you may hear different terms used to describe each type of childhood arthritis. For clarification on the new international "juvenile idiopathic arthritis" classification, as compared with the American "juvenile rheumatoid arthritis" and the European "juvenile chronic arthritis," see classification.