The reactive hemophagocytic syndrome comes from an overstimulation of the immune system which causes a cytokine storm. This is a life-threatening condition caracterised by a febrile cytopenia, hepatosplenomegaly and multi-organ failure. The diagnosis is not easy and the HScore can be useful, looking at hyperferritinemia, hypertriglyceridemia, and hypofibrinogenemia. The evidence of hemophagocytosis in the bone marrow is not necessary nor sufficient to make the diagnosis but is part of the workup. The underlying cause has to be actively sought, typically an infectious, malignant or autoimmune disorder. This syndrome should be supported in conjunction with the hematologist, and initiation of a treatment is a medical emergency.