<p>  Myelin oligodendrocyte glycoprotein antibody (MOG-Ab) is identified in some patients with ADEM. Whether serial change of MOG-Ab titer is useful for deciding on the therapeutic strategy for multiphasic ADEM has not been clarified. We report the case of multiphasic ADEM who was treated with intravenous immunoglobulin (IVIG) therapy and whose MOG-Ab was serially evaluated. The patient is 9-year-old boy. He showed acute onset of headache and delirium. T2-weighted and fluid attenuated inversion recovery (FLAIR) magnetic resonance imaging (MRI) showed multiple hyperintense lesions in bilateral subcortical and cortical areas, and we diagnosed the patient with ADEM. Although symptoms and MRI lesions were resolved by steroid pulse therapy, he experienced three time relapses characterized by headache and abnormal behavior as well as new MRI lesions during tapering of the therapy. We started monthly IVIG from 5 months after onset, and no relapse of ADEM was encountered. MOG-Ab titer ranged from 4,096 to 2,048 before IVIG therapy. After starting IVIG, MOG-Ab titer gradually decreased. We were able to decrease and discontinue oral steroids. After steroid discontinuation, MOG-Ab titer remained low. We started to decrease the dose of IVIG at 1 year and 3 months after the onset of ADEM and terminated IVIG at 1 year and 9 months after onset by checking low titer of MOG-Ab. Our results suggest that serial evaluations of MOG-Ab may contribute to the monitoring of disease activity and decision-making on drug tapering and termination for multiphasic ADEM. The utility of serial evaluations should be validated in studies of larger numbers of patients.</p>