We present a case of a 17-year-old girl with sensory disturbances and numbness in the left upper limb, left lower limb, and the part of face and tongue. Brain magnetic resonance imaging revealed an irregular area with high-signal intensity on T2 sequence and FLAIR sequence placed in the lower part of the right thalamus, right cerebral peduncle, right substantia nigra, and lateral part of the red nucleus. Differential diagnosis included a demyelination process and inflammatory process caused by infectious factors and the autoimmune process. The pathologic area decreased after high-dose methylprednisolone therapy. Ultimately, we made a diagnosis of clinically isolated syndrome.