Familial hemophagocytic lymphohistiocytosis (FHL) is probably a genetically transmitted disease affecting infants and very young children. Cardinal symptoms are fever, hepatosplenomegaly, and pancytopenia. Frequently meningeal involvement is seen, manifested by neurologic symptoms and a lymphohistiocytic pleocytosis with increased protein levels in the cerebrospinal fluid. Characteristic laboratory findings in FHL are hypertriglyceridemia and hypofibrinogenemia, which are reversible with treatment. The disease has been rapidly fatal in most patients, but recently longterm remissions have been achieved with cytotoxic agents. Pathohistologic examination shows a widespread infiltrate of lymphocytes and mature macrophages with prominent hemophagocytosis affecting especially liver, spleen, lymph nodes and the central nervous system. Atrophy of the lymphatic tissue is a common finding. From the histologic picture FHL has to be grouped among the histiocytoses of reactive origin since the cells involved show no signs of malignancy. The etiology and pathogenesis of FHL are not known at present. Immunologic studies present evidence for a disturbed function of T lymphocytes, but a secondary immune defect seems to be more likely than primary immune deficiency. Among the broad clinical spectrum of histiocytic disorders especially histiocytic reactions due to infection, histiocytosis X and malignant histiocytosis have to be considered in the differential diagnosis of FHL.