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A 49-year-old woman with a history of adult-onset Still's disease (AOSD) presented with fever, general malaise and a rash. Laboratory blood testing revealed an extremely high level of serum ferritin, essentially restricting the differential diagnosis to either haemophagocytic syndrome as a complication of AOSD, or a flare-up of the latter. Haemophagocytosis as a complication of AOSD was diagnosed in our patient. After treatment with prednisone, she fully recovered and the serum ferritin returned to a normal level. Haemophagocytic syndrome is a rare but potentially life-threatening complication of lymphoproliferative and autoimmune diseases, as well as of viral infections. It is characterised by high fever, hepatosplenomegaly, cytopenia and extremely high levels of serum ferritin. Activation of macrophages and histiocytes induces phagocytosis of erythrocytes in the bone marrow and other parts of the reticuloendothelial system. The fact that haemophagocytic syndrome and AOSD are often described together, and coincide with extremely elevated serum ferritin levels characteristic to both entities, suggests a related pathogenesis.