Juvenile idiopathic arthritis (JIA) is one of the most common childhood rheumatic diseases. Clinically, it is defined as arthritis of unknown origin that starts before the age of 16, and persists for at least 6 weeks. Next to a certain genetic predisposition, environmental factors play a role leading to a chronic inflammatory response. JIA is not a single disorder but consists of a heterogeneous group of auto-immune inflammatory diseases. It has variable rates in course and activity of disease. Based on 6 months of clinical symptoms and global prognostic factors, the following clinical subtypes of JIA are recognized: systemic JIA, oligoarthritis, RF-negative polyarthritis, RF-positive polyarthritis, psoriatic JIA, enthesitis- related arthritis, and undifferentiated arthritis. Systemic JIA, also known as Still's disease, is a subtype with strong systemic clinical symptoms. Patients with systemic JIA have, in addition to arthritis, prominent symptoms of systemic inflammation such as spiking fever, rash, pericarditis, peritonitis, lymphadenopathy and organomegaly. A severe and often life-threatening complication occurring in 10-30% of patients with systemic JIA is macrophage activation syndrome. Polymorphisms in the IL6 and in the MIF gene have been found to be associated with susceptibility to the disorder. Based on the known relevance of IL6 in JIA pathophysiology, tocilizumab has been investigated and approved for use in the treatment of systemic and polyarticular JIA.