Primary central nervous system lymphomas constitute less than 2% of primary brain tumors. Although their cause is unknown, they are in some way related to immunosuppression. The typical patient is a middle-aged man who displays the syndrome of a subacute mass lesion. These tumors most commonly occur as a single, bulky mass in the hemispheric white matter or the deep gray matter, but multiple tumors occur in approximately a fourth of the patients. The findings on computed tomography are distinctive and nearly pathognomonic. The majority of these tumors are B-cell lymphomas with aggressive histologic changes. Surgical intervention should be reserved for tissue diagnosis because surgical removal does not seem to influence the outcome. Conventional therapy consists of whole-brain irradiation with or without chemotherapy. The prognosis for patients with such tumors is grim, the median duration of survival being less than 2 years with conventional therapy and the 5-year survival being less than 5%.