Hemophagocytic lymphohistiocytosis is a life-threatening condition associated with multiple organ dysfunctions. This entity is related to inappropriate stimulation and proliferation of cytotoxic lymphocytes and macrophages inducing phagocytosis of blood cells. Hemophagocytic lymphohistiocytosis should be considered rapidly in any unexplained febrile cytopenia. Biological markers are high ferritin and triglyceride levels, and low fibrinogen. Hemophagocytic lymphohistiocytosis diagnosis should not be ruled on or out solely on the presence or absence of hemophagocytosis features on smear or biopsy sampling. It is either "primary/genetic" (pediatric or familial disorders) and characterized by a lack of intrinsic cytotoxicity of NK cells or T CD8 lymphocyte, or "secondary/reactive" due to malignancy, infectious or autoimmune origin. Mortality is 50% (including all etiologies), and this severity requires rapid and "aggressive" investigations with multidisciplinary approach including intensive care unit team. The immediate aim of therapy is suppression of the severe hyper-inflammation, which can lead to multiple organ failure. Emergency treatment is currently based on etoposide (VP16), pending to the identification and treatment of underlying cause.