This review of multicentric reticulohistiocytosis is based upon study of one new patient and three previously reported from this institution. In the literature, there are now 23 well-documented examples. This is a systemic disease in which weight loss and episodes of pyrexia occur frequently. Lesions have been demonstrated in skin, mucosa, subcutaneous tissues, voluntary muscle, synovia, bone and periosteum, lymph nodes, marrow, larynx, and endocardium. Evidence of pulmonary involvement has been noted only in women. Although evidence of tuberculosis occurs in approximately one-fifth of patients with this disease, it is likely that tuberculosis is coincidental. Despite thorough investigation of one patient, there was found no evidence of an immunologic deficit. Intensive histochemical study of skin, mucosa, and synovia of the four patients showed the principal finding to be strongly positive periodic acid-Schiff (PAS)-reactive material in histiocytes and giant cells. The material is a sort of mucin, mucoprotein or glycoprotein. The results of this study indicate that the disease is not an essential disorder of lipid metabolism and not a rheumatic disease. The exact nature of the condition is not known. It is manifest by proliferation of abnormal cells of histiocytic origin, which secondarily ingest a variety of substances, including PAS-reactve material, iron, melanin, and lipid. The name "multicentric reticulohistiocytosis" is preferred.