Until recently, malignant histiocytosis was a clearly defined clinical entity marked by fever, progressive wasting, lymphadenopathy, hepatosplenomegaly, and pancytopenia. However, for many years the morphologic findings in this disease continued to cause a great deal of controversy. Now it seems clear that most cases of malignant histiocytosis represent anaplastic large cell lymphoma (ALCL) with Ki 1 expression, and they are not related to the monocyte/macrophage system. This conclusion is based on histopathologic and immunohistochemical findings, and more recently, on results from genotypic studies. Thus, malignant histiocytosis is a "vanishing disease."