Thirty-five patients diagnosed with "malignant histiocytosis" from 1984 to 1989 were studied for clinical, laboratory, histopathological features as well as survival and response to therapy, Immunocytochemistry and immunophenotypic studies were performed in 12 cases using the paraffin immunoperoxidase method. The staining included alpha-1 antichymotrypsin, muramidase, immunoglobulins and monoclonal antibodies specific for T, B lymphocytes and macrophage. From the clinical features, responsiveness to therapy and survival, the patients were divided into 2 groups: the non-responders (25 cases) and responders (10 cases) groups. Very short median survival of 1.25 months was found in the non-responders, whereas, longer median survival of 14.15 months was found in the responder group. Important different clinical and laboratory features were observed among these two groups. Unresponsiveness to treatment; rapidly progressive pancytopenia, increased hemophagocytosis, presentation of immature cells in blood with extensive infiltration of malignant cells in the bone marrow; severe jaundice and deterioration of hepatic function accompanied by early extranodal involvement were almost exclusively observed initially in the non-responder group. Satisfactory response to treatment was observed only in the responder group. Similarity of histopathology, cytology and immunophenotype was observed in these two groups. The immunophenotypic study in 12 cases showed 5 cases of B-cell lymphoma, 3 cases of T-cell (with 1 Ki-1 -positive) lymphoma; 1 case of Ki-1 positive non-T, non-B anaplastic large cell lymphoma; and 3 cases of undetermined cell lineage. From this study, so-called "malignant histiocytosis" appears to be a disorder of heterogeneity. The immunophenotypes of malignant cells indicated that their origin belonged mostly to lymphoid cell lineage. Based on their clinical feature of the early hematogenous spread along with the distinct histopathological and immunophenotypic findings, the term "pleomorphic large cell hematolymphoma" is proposed to be used instead of the old misnomer, "malignant histiocytosis" (MH).