Malignant lymphoma of the central nervous system (CNS) have exhibition a marked reponse to radiation therapy so that early diagnosis may be of nalue. the authors reviewed 19 patients with diffuse histiocytic lym-phoma of the brain including 13 with primary disease and 6 with secondary disease. the results of treatment with radiation theraphy(RT) and/or radiation therapy followed by intrathecal methotraxate (MTx) were analyzed. Both primary and secondary lymphomas exhibited the characteristic CT appearance a large solid homogenously enhanced mass with varying amount of edema. Multifocal lesions were also seen in primary (7.13) and secon-dary lymphoma (2/6) Secondary lymphoma affected the leptomeninges and subcortical white matter which was contrasted the involvement of deep cerebral nuclei and periventricular white matter in primary lym-phoma. The recurrence rate was higher in patients treated with RT alone (2/6) than RT and intrathecal MTx (1/11) but necrotizing leukoencephalopathy was developed in 54.5% (6/11) of latter group.