A case of primary cerebral malignant lymphoma associated with hydrocephalus is reported. The patient was a 54 year-old male who enjoyed good health until the onset of headache and vomiting 4 weeks before admission. His consciousness was alert and neurological examination revealed severe papilloedema with retinal hemorrhage. No lymph node or abdominal tumor enlargement were noted. CT scan and MR images revealed no abnormal lesion except mild ventriculomegaly. CSF study revealed mild elevation of protein and sugar and cell count was 66/3. CSF cytology revealed atypical lymphoid cell with irregular nuclear contour and large nucleolus. Immunological marker studies of the tumor cell revealed increasing of anti J-5 (CD10), anti B-4 (CD19) and OKT-IA1. The patient was treated by a whole brain irradiation and chemotherapy after V-P shunt. It is 12 months since the operation, and the patient's condition is still good.