Familial hemophagocytic lymphohistiocytosis (FHL) is characterized by proliferation and infiltration of hyperactivated macrophages and T-lymphocytes manifesting as acute illness with prolonged fever, cytopenias, and hepatosplenomegaly. FHL presents itself within the first months or yearsof an infant, on occasion, in utero, although later childhood or adult onset is more common. There may be present neurologic abnormalities initially or may develop later. They may include increased intracranial pressure, irritability, neck stiffness, hypotonia, hypertonia, convulsions, cranial nerve palsies, ataxia, hemiplegia, quadriplegia, blindness, rash and lymphadenopathy are common. Other effects include liver dysfunction and bone marrow hemophagocytosis. The chance of survival of children with typical FHL, without treatment is less than two months. Progression of hemophagocytic lymphohistiocytosis and infection account for the majority of deaths in untreated infants.