Monocytes and macrophages are closely related in origin, structure and function. True histiocytic lymphoma is a neoplasm of phagocytic histiocytes and is treated as non-Hodgkin's lymphoma. Tissue involvement with myeloid leukemia, including monocytic leukemia, is called extramedullary myeloid cell tumor or granulocytic sarcoma and treated with antileukemic chemotherapy. Differentiating true histiocytic lymphoma from tissue involvement with monocytic leukemia is a rather impossible task using available morphologic, cytochemical, immunocytochemical and molecular methods. The two entities need to be unified probably more appropriately as extramedullary myeloid cell tumor of monocytic origin, and should therefore be treated as such.