We report the case of a patient who developed extensive brain lesions during fingolimod (FTY720) treatment in the TRANSFORMS study. His initial diagnosis was multiple sclerosis, but after encephalopathy anti-aquaporin4 antibody (anti-AQP4 Ab) was detected, it was changed to neuromyelitis optica spectrum disorder. After treatment with fingolimod, he developed bilateral extensive brain lesions. The brain MRI showed lesions predominantly involving the right frontal and parietal lobes, with vasogenic edema and enhancement. He had residual encephalomalacia and no recurrence with steroid treatment over 3 years following withdrawal of fingolimod.