A 79-year-old, bedridden woman with an untreated colon cancer, developed abrupt disturbance of consciousness and high fever. Brain MRI showed significant high signals in the subarachnoid space and ventricles in diffusion-weighted images (DWIs), and she died on the same day. At autopsy, much exudate was found over the base of the brain, leading to a diagnosis of purulent meningitis. On histological findings, inflammatory cell infiltration was significantly restricted within the subarachnoid space, but not into the brain parenchyma. This case report demonstrated that high signals in subarachnoid space and ventricles in DWIs may be very useful for diagnosis of purulent meningitis.