Morphological diagnostic criteria for the various “histiocytoses” must be reevaluated in the context of new concepts of lymphology and the characteristics of a unique histiocyte, the Langerhans' cell. Indeed, many diseases categorized as histiocytoses are now known to be proliferative disorders featuring the lymphocyte with a histiocytoid appearance. The term lymphoreticulosis is preferred in reference to the various disorders featuring proliferation of lymphoid and reticuloendothelial cells considered in this paper.There are numerous reactive syndromes resulting in lymphoreticulosis. Careful evaluation of the patient is required for the identification of etiological agents and therapy should be directed in light of etiology.Patients with deficiency of defense systems may present with lymphoreticulosis, the signs and symptoms of which may overshadow the primary host defense deficiency state.A group of diseases featuring primary proliferation of Langerhans' cells, unique histiocytes with characteristic ultrastructural morphology, is the major subject of this seminar and encompasses the histiocytosis × spectrum of disease (eosinophilic granuloma, multifocal, and systemic disease).Disorders within this category, mainly the multifocal and systemic diseases, pose serious problems in terms of variation of clinical course and prognostic indicators which make clinical management difficult.An entity not to be confused with primary Langerhans' cell proliferative disorder is a highly lethal and most often familial disease designated lymphohistiocytic reticulosis with phagocytosis. Specific diagnostic criteria are posed and include bone marrow, cerebrospinal fluid, and hepatic morphological changes.Other lymphoreticuloses not readily classifiable are described briefly.It is suggested that the rarity and complexity of these disorders demand that children manifesting signs and symptoms of the lymphoreticuloses be promptly referred to a sophisticated pediatric hematology-oncology center for proper patient care and productive investigation.