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Macrophage activation syndrome (MAS) is a severe, potentially fatal complication of systemic juvenile idiopathic arthritis (sJIA) with features of hemophagocytosis, leading to coagulopathy, pancytopenia, and liver and central nervous system dysfunction. MAS is overt in about 10% of children with sJIA but may occur subclinically in another 30-40% of patients. The diagnosis of MAS in sJIA can be challenging because it may mimic a flare of the underlying disease or be confounded with an infectious complication or a medication side effect. So far, universally agreed upon diagnostic criteria for MAS are not available. A multinational collaborative effort is under way that is aimed to generate new criteria for diagnosing MAS as a complication of sJIA. The first step of the project has led to the identification of, by means of a Delphi survey, the clinical, laboratory and histopatho logic features of MAS that were felt to be most important by a large sample of international pediatric rheumatologists with experience with MAS in children with sJIA. The diagnostic performance of potential criteria will be scrutinized further in the second phase of the study, through the analysis (ongoing) of real patient data. The final set of criteria is meant to be established in a consensus conference of experts using a combination of statistical and consensus formation techniques. The ultimate goal of the project is to develop a core set of criteria that is both highly sensitive and specific. These criteria will assist physicians in MAS diagnosis in children with sJIA and enable quick institution of appropriate therapy.