Abstract: Objective: To investigate the clinicopathologic features and histopathologic types of the so called malignant histiocytosis (MH) and to investigate its cell origin and relation with Epstein-Barr virus. Methods: Paraffin sections of 13 cases of M H diagnosed previously based on clinical and morphologic features were analyzed with morphological and immunohistochemical methods. Results:Based on the morphological features, 13 cases of M H can be divided into three types: Medium-small sized lymphoid cell type; Atypical histiocytes type; Hodgkin's -like type. The immunohistochemical study of 13 cases showed 9 cases of T-cell lymphoma that expressed CD45R0, 2 cases of histiocytic lymphoma that expressed CD68, and 1 case of hybrid form (CD45R0 and CD68 coexpressed), 1 case of anaplastic large cell lymphoma that expressed CD45R0, EMA, and CD30, none of them originated from B or 5K-cell that expressed CD20 or CD56. 0f the 13 cases MH, there were 7 cases expressed L M P -1, in which 5 cases belonged to T-cell lymphoma (5/9), 1 case histiocytic origin (1/2) and 1 case anaplastic large cell lymphoma. Conclusions: Classifying M H by histological features is important in clinic pathology for identifying the cell origin of tumor. The so called malignant histiocytosis, in fact, is a group of malignant heterogeneous tumors. The true histiocytic origin is believed extremely rare and the diagnosis oi it should be based on the very strict standard. Some of the malignant histiocytosis shows a relationship with E B : infection.