Eighty-nine cases variously diagnosed as histiocytosis X, eosinophilic granuloma, Hand-Schuller-Christian disease, Letterer-Siwe disease, or malignant histiocytosis from one institution were reviewed. Follow-up data were obtained for all patients. On the basis of clinicopathologic correlation, the following distinct groups were identified. 1. Disseminated histiocytosis (12 patients) with involvement of more than two organ systems at the time of recognition of the disease, affecting children less than 3 years of age (the patient's general condition is poor and hepatosplenomegaly is common. The patients do poorly, and all the patients in this group of the present study died. 2. Histiocytosis confined to bone, whether monostotic (36 patients) or polyostotic (eight patients). This type is self-healing, does not disseminate, and does not require intensive therapy. Clinically the patients are older and in good general condition during the course of the disease. Histologically histiocytes in these two groups are cytologically benign, and the whole process is inflammatory rather than neoplastic. Presence of giant cells, eosinophils, and necrosis are usually associated with better prognosis. Many neoplastic diseases and infectious processes manifest as histiocytosis. More than a third of the cases of this study diagnosed as histiocytosis proved to be other processes. This confusion contributes to differences of various reports and difficulty of interpretation of findings.