Reactive haemophagocytic syndrome is characterized by activation and uncontrolled non-malignant proliferation of T lymphocytes and macrophages, leading to a cytokine overproduction, which accounts for the main biological signs. Children usually present with an acute febrile illness, fulminant pancytopenia and hepatosplenomegaly, posing a problem of differential diagnosis with severe sepsis. Hemopoietic cells are actively ingested by monocytes/macrophages in various organs, including lymph nodes, bone marrow, liver and spleen. This exarcerbation of the histiocytic system is currently classified among the reactional histiocytoses. It reflects an inappropriate host immune response. Most patients have a known underlying disease (hemopathy, lupus, systemic juvenile arthritis, HIV infection). In the few cases that occur in the apparent absence of any risk factors, investigations should be made to look for predisposing inherited diseases, such as familial lymphohistiocytosis or Purtilo's disease in boys. The treatment rests on immunosuppressive agents, followed by bone marrow transplantation, which can provide a definitive cure in genetic forms.