Twenty-two children with malignant histiocytosis ranging in age from 2 to 13 1/2 years are described. In 10 cases, the correct diagnosis was only made retrospectively. The most salient pathologic features were found in lymph node biopsy specimens. Fever and wasting were the most prominent symptoms in 21 cases, and abdominal pains were striking in 9. Peripheral lymphadenopathy, mostly of the cervicoaxillary type, was present in 21 patients, with marked tenderness in 14. Abnormal mediastinal and/or paraaortic lymph nodes were detected radiographically in two-thirds of the patients. Other prominent features included subcutaneous inflammatory infiltration in 12 patients and skin nodules in 8. Pleural effusions were seen in 8 children. The haematologic findings are described. Only 5 of 19 children had bonemarrow involvement. Fifteen patients died, one to 45 months from onset of their illness (median survival time, five months). Nine patients are alive with no evidence of disease 21 to 46+ months (median, 40 months) after the time of diagnosis and 8 have currently been off treatment for periods ranging from three to 29 months; they all have been treated with vincristine, prednisone, cyclophosphamide, and adriamycin and 4 have achieved remission after treatment with vinblastine, bleomycin, and CCNU or cytosine-arabinoside. It is concluded from this study that clearly delineated clinical features of malignant histiocytosis in childhood should allow more rapid determination of the proper diagnosis and should result in early treatment by means of intensive systemic combination chemotherapy, which has dramatically improved the prognosis.