A 46-year-old female presented with a 1-week history of mental change, confusion and headaches. Investigations revealed evidence of sterile meningitis. CAT scanning of the brain demonstrated marked contrast enhancement around the ependyma, and later examinations showed extension of the process deep into the white matter, Cerebral biopsies were non-diagnostic and, despite ventricular drainage and treatment with antibiotics and high dose steriods, the patent died. At postmortem there was extensive tumour tissue distributed in a butterfly shape around the ventricles, and microscopy revealed typical apperances of primary CNS lymphoma. On the basis of the experience of this case and a review of recent literature, it is suggested that the CAT scan appearances of this tumour are quite typical, and that cranial irradiation may be justified in the absence of specific histological diagnosis.