Malignant histiocytosis is a systemic, progressive, invasive proliferation of morphologically atypical histiocytes and their precursors. Many different terms have been used by different authors. Synonyms are ""Leukemic reticuloendotheliosis, Histiocytic leukemia, Malignant leukemic reticulohistiocytosis, and Histiocytic medullary reticulosis."" In December 1974, a 47-year old man developed a generalizaed weakness, anorexia, malaise, intermittent fever and enlarged cervical lymph node. These were followed by enlargement of the lymph nodes in the groins, neck and axilla. The liver and spleen were enlarged. The neck biopsy showed a highly atypical histiocytic proliferation, frequent erythrophagocytosis and moderate frequent mitotic figures. Irradiation amounting to 2,250 rads was administered to the cervical area. The local mass disappeared but the general condition was not improved.