Macrophage activating syndrome (MAS) being a reactive form of haemophagocytic lymphohistiocytosis (HLH) is increasingly diagnosed complication of rheumatic diseases including juvenile idiopathic arthritis, adult-onset Still’s disease, and systemic lupus erythematosus. Both MAS and familial HLH are associated with impaired activity of NK cells resulting in increased activation and proliferation of macrophages with excessive cytokine production and organ infiltration. Typical symptoms include fever, organomegaly, and cytopenia being a consequence of haemophagocytosis. Increased ferritin level, hypofibrinogenemia, and hypertriglyceridemia are useful laboratory markers. The management comprise elimination of the triggering factor, i.e. usually viral infection, high-dose glucocorticosteroids, cyclosporine A, intravenous immunoglobulins, and etoposide.