Between 5% and 10% of children with systemic or polyarticular onset juvenile idiopathic arthritis (JIA) do not response adequately to treatment with non-steroidal antiinflammatory drugs and immunosuppressive drugs such as methotrexate and corticosteroids. Among such children severe joint destruction, growth retardation, drug toxicity, and psychological distress are common and some (around 2–4% of children with systemic onset disease) die of the disease. The main causes of death are drug toxicity, infection, amyloidosis, and the later development of diseases such as systemic lupus erythematosus or scleroderma. Anti-tumour necrosis factor (anti-TNF) treatment has proved effective for children with polyarticular onset disease but less so in …