Hemophagocytic lymphohistiocytosis, terminology that designates a syndrome that may be familial or sporadic, with or without an associated viral infection, is presented as the prototype of a hemophagocytic syndrome, a condition in which there is uncontrolled activation of the cellular immune system. Diagnostic criteria include idiopathic fever, splenomegaly, cytopenias, hypertriglyceridemia, hypofibrinogenemia, and the presence of hemophagocytosis. The surgical and autopsy pathology features infiltrates composed of lymphocytes and ordinary, but activated, histiocytes and hemophagocytosis. The chronic hepatitis-like hepatic lesion is noted to be characteristic, if not unique, in this age group and setting. Current concepts of pathophysiology focus on the role of cytokines, particularly interleukin (IL)-1, IL-2, soluble IL-2 receptor, plasminogen activator, and prostaglandins. The clinicopathologic features of the syndrome can be accounted for by the uncontrolled and unopposed production and release of these mediators. Nosology places hemophagocytic lymphohistiocytosis in the position of the most important of the "benign" histiocytosis syndromes that involve ordinary histiocytes of the mononuclear phagocytic system in contrast to Langerhans cell histiocytosis (histiocytosis X) in which pathological dendritic histiocytes are operative. Features that distinguish hemophagocytic lymphohistiocytosis from other disorders, such as malignant histiocytosis, X-linked lymphoproliferative disorder, congenital immunodeficiency states, the accelerated phase of Chediak-Higashi syndrome, and cytophagic histiocytic panniculitis, which may be associated with a hemophagocytic syndrome, are presented.