Macrophage activation syndrome is due to macrophage stimulation secondary to excessive cytokine secretion. Infectious (mainly viral) and neoplastic (lymphomas primarily) diseases are the mainly triggering circumstances of this syndrome associated with immunodepression. Signs are fever, hepatosplenomegaly, lymphadenopathy, cytopenia, deranged liver function increases level of lactate dehydrogenase, triglyceridemia and serum ferritin. Diagnosis is established by the presence of cellular phagocytosis by macrophage on medullary or hepatic biopsy. It is a potentially life threatening condition. Treatment of the triggering circumstance associated with corticosteroids, chemotherapy (etoposide) or intravenous immunoglobulins, according to etiology, are necessary.