A 23-year-old female patient with primary vasculitis of the central nervous system simulating a brain tumor is described. The clinical picture was represented by migraine-like headaches, ataxia, transient numbness of the right leg, the lips, double vision, a slight decrease of cognitive functions. MRI of the brain revealed a tumor-like focus in the cerebellum, intensively accumulating contrast, containing micro-hemorrhages (SWI mode). Small single ischemic foci in the brain hemispheres and brain stem were also found. MR angiography (3T) did not found any pathology. Examination of the cerebrospinal fluid revealed a small cytosis (mainly T-lymphocytes) and a slight increase in protein. The results of the analysis of cerebrospinal fluid for syphilis, tuberculosis and the herpetic group of viruses were negative, type 1 oligoclonal synthesis was found. Blood tests for toxoplasmosis, antibodies to aquaporin, anti—neutrophil antibodies, markers of systemic inflammation were within normal limits. Different diagnoses were assumed: demyelinating disease, encephalitis, multiple encephalomyelitis, lymphoma. The diagnosis was established only by a brain biopsy — lymphocytic vasculitis was revealed. According to the immunohistochemical study, T-helpers predominated in the infiltrates. After pulse therapy with Metylprednisolon (1000 mg intravenously drip №. 5), the patient’s condition almost returned to normal. It was recommended to take prednisolone per os (starting dose 60 mg) for 7 months.