Systemic juvenile idiopathic arthritis (JIA) constitutes about 10% of cases of childhood-onset idiopathic chronic arthritis. Chronic arthritis develops in up to 50% of cases and conventional disease-modifying antirheumatic drugs (DMARDs) such as methotrexate and tumour necrosis factor inhibitors may be relatively ineffective. Prolonged steroid treatment may be needed with consequent risk of osteoporosis, fractures and small stature. Up to 75% of patients may need joint replacement as adults. New treatments …