Hemophagocytic syndrome (HS) is an uncommun clinicobiologic entity characterized by systemic proliferation of benign monocyte-macrophage cells. Two forms have been described : familial hemophagocytic lymphohistiocytosis (primary HS) and reactive (secondary) HS. The underlying diseases are infection (mostly viral), neoplasia, systemic lupus erythematosus and Still's disease, or drug induced HS. Pathogenesis is still poorly undestood, but macrophage and lymphocytes are the major actors. The clinical and biological findings are related to the uncontrolled secretion of cytokines. Mortality rate is high and the treatment should be rapidly symptomatic and directed to the underlying disease.