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Fever of unknown origin in a patient of systemic onset juvenile idiopathic arthritis.

Hemophagocytic lymphohistiocytosis is a potentially fatal condition characterized by pathologic immune activation, which can complicate infections, childhood systemic rheumatologic diseases and malignancies. Here we report a case of reactive hemophagocytic lymphohistiocytosis [macrophage activation syndrome] complicating systemic onset juvenile idiopathic arthritis, which was treated successfully with dexamethasone and cyclosporine. Reactive hemophagocytic lymphohistiocytosis or macrophage activation syndrome should be considered in patients of juvenile idiopathic arthritis with prolonged fever of unknown origin and cytopenias. Early diagnosis with high index of suspicion and prompt, aggressive treatment are needed for successful outcomes.