Primary central nervous system(CNS) lymphoma is an uncommon neoplasm which is considered to be radiosensitive. We have recently experienced one of such cases with a history of systemic lymphoma(diffuse histiocytic lymphoma) of the left cheek and left forearm in a 56-year-old man. Chief complaints include headache and generalized weakness for 1 month. MRI(magnetic resonance imaging) revealed an enhancing irregular mass lesion in both the basal ganglia and corpus callosum with minimal surrounding brain edema. CSF analysis and cytology revealed no specific abnormal findings. Without brain biopsy, we concluded that the lesion was a primary CNS lymphoma based on the fact that the patient have had a history of diffuse histiocytic lymphoma and MRI findings were compatible to primary CNS lymphoma. He was treated with dexamethasone for 2 weeks and 3900 cGy radiation therapy for 5 weeks. He improved gradually, and control MRI after radiotherapy showed complete resolution of the lesion.