Reactive macrophage activation syndrome or haemophagocytic lymphohistiocytosis is a life-threatening immunoregulatory disorder characterized by the activation of the mononuclear phagocytic system with prominent haemophagocytosis particularly involving the bone marrow, reticuloendothelial system and central nervous system. It frequently occurs in children as a familial autosomal recessive disorder and it has been reportedly associated with infections, underlying hematopoetic malignancies or autoimmune disorders. It has been suggested that central to the pathogenesis of this disorder is uncontrolled T-cell activation which results in the excessive secretion of cytokines. The clinical and laboratory findings are dramatic, at presentation with persistent fever, hepatosplenomegaly, profound depression of three blood cell lines, elevated serum liver enzyme values, coagulopathy and abundant haemophagocytosis can be visualized in bone marrow. Most of the described clinical findings could be explained by the oversecretion of proinflammatory cytokines and the presence of histiocytic infiltrates. In general, immunosuppressive therapy, cytotoxic chemotherapy with immunomodulatory agents will induce remissions in a substantial percentage of patients, whereas others require allogenic bone marrow transplantation.