We reported a patient with immune-mediated encephalopathy showing refractory epilepsy and multiple brain lesions on MRI. The patient had high titers of anti-glutamic acid decarboxylase (GAD) antibody in sera and cerebrospinal fluid (CSF). A 36-year-old previously healthy woman was admitted to our hospital with onset of sudden generalized seizure that then persisted for one month. She had repeated epileptic attacks accompanied with loss of consciousness, and was refractory to valproic acid, zonisamide (200 mg/day) and phenobarbital (200 mg/day). Brain MRI showed multiple hyperintense lesions in predominantly bilateral frontal lobes, parietal lobes, occipital lobes and cingulate cortices. EEG showed epileptic activities (frequent sharp waves) in bilateral frontal regions. After admission, attacks disappeared through the administration of clonazepam (1.5 mg/day), though the patient remained slightly disoriented. As titers of anti-GAD antibody in sera and CSF were extremely high, we implemented plasma exchanges. After treatment, titers of anti-GAD antibody in sera and CSF decreased. The patient completely recovered to an alert state and the abnormal MRI lesions almost disappeared. Since GAD catalyzes production of gamma-aminobutyric acid (GABA), it is proposed that anti-GAD antibodies reduce synthesis of GABA or interferes with exocytosis of GABA in the nervous system. Anti-GAD antibodies are detected in some rare neurological disorders such as stiff-person syndrome. Recently, anti-GAD antibodies have been reported as implicated in cerebellar ataxia, palatal myoclonus, refractory epilepsy and limbic encephalitis. Epilepsy associated with the anti-GAD antibody is mostly pharmacoresistant temporal lobe epilepsy; with brain MRI showing no abnormality or only hippocampal sclerosis. It is very rare that brain MRI shows extensive abnormal lesions except in the hippocampus. This case suggests that anti-GAD antibodies could contribute to unexplained encephalopathy with extensive brain MRI lesions and drug-resistant symptomatic epilepsy.