A 74-year-old woman presented with progressive mental deterioration following a low-grade fever and headache. Upon admission, she appeared lethargic, could not obey simple commands and was disoriented to time, place and person. She had low-grade fever and mild neck stiffness. The cerebrospinal fluid had an elevated protein content of 496 mg/dl, contrast-enhanced MRI revealed diffuse leptomeningeal enhancement, particularly in the occipital area, and a cerebral angiogram showed diffuse segmental narrowing of multiple intracranial arteries, especially in the distal portion of the right middle cerebral artery. A clinical diagnosis of granulomatous angiitis of the central nervous systems (GANS) was made, and corticosteroid therapy was initiated. The patient improved gradually, and corticosteroid therapy was tapered to the maintenance dose (prednisolone 0.4 mg/kg daily). An open brain biopsy showed multiple vessels containing granulomatous inflammation with giant cells. GANS is one of the most challenging neurologic disorders to diagnose because of its relative rarity and the lack of specificity of clinical signs and efficient, non-invasive, diagnostic tests. In this case, we were able to begin corticosteroid therapy in the early stage of the disease (before brain biopsy), and it yielded a good outcome.