Twenty-six cases of histologically proven malignant histiocytosis are presented. Patient ages ranged from 20 months to 82 years, and a 10:3 male preponderance was seen. Clinical presentation is similar to that of malignant lymphoma and leukemia; histologic differentiation is required. The frequency of intrathoracic involvement at the time of presentation is similar to that of non-Hodgkin's lymphoma. Retroperitoneal lymph node invasion was readily seen on five of the eight lymphograms performed. Hepatosplenomegaly was a common finding, and three of the patients had evidence of bone involvement during the course of their illness. While there are no radiographic findings specific to malignant histiocytosis, the presence of hepatomegaly should suggest this possibility. The establishment of the correct diagnosis is important to guide choices in chemotherapy.