Clinicopathological analysis was performed in 19 patients diagnosed clinically with malignant histiocytosis. Ultimately, 9 patients died and 10 are still alive. All 19 had fever of unknown origin. Among the 10 surviving patients, 6 recovered with only supportive therapy such as antibiotic treatment. One recovered with steroid therapy and 2 with VP (vincristine and prednisolone) therapy. Complications due to immunodeficiency were detected in one surviving patient and 2 who died. All 9 patients who died had anemia, and 8 had thrombocytopenia. However, among survivors, only one had anemia and only 2 had thrombocytopenia. Chromosomal abnormality was detected in one patient who died. Histiocytic cells were classified morphologically into 3 types: immature, intermediate and mature. In 4 patients who died, histiocytic cells were immature, but in 4 others mature histiocytic cells were detected. In 5 of the 10 surviving patients, histiocytic cells were of the immature type. Immuno-histochemical analysis of the origin of histiocytic cells in 8 deceased patients showed T-zone histiocytes in one, T cells in one, monocyte phagocytic system (MPS) in 5, and histiocytes of unknown origin in one. Thus, malignant histiocytosis is a heterogenous entity including reactive histiocytic disorder, lymphocytic neoplasm and true histiocytic neoplasm. In histiocyte proliferative disorders, red blood cell counts and platelet counts are useful for assessing prognosis, while cytological findings only confuse this evaluation.