A 69-year-old woman was admitted due to gradual progression of daytime sleepiness and forgetfulness over a period of approximately 1 month. Bradycardia and hypothermia were observed on admission, and neurological examination revealed memory disturbance, mild dysarthria, and bradykinesia. Fluid-attenuated inversion recovery (FLAIR) images of the brain magnetic resonance imaging (MRI) indicated signal hyperintensity in the region bordering the lateral and third ventricles. Serum anti-aquaporin 4 (AQP4) antibody was detected. The patient had no history or findings of optic neuritis or myelitis, and she was diagnosed as anti-AQP4 antibody-associated disorder. Diencephalon lesion and/or symptoms are rarely observed at the onset of neuromyelitis optica. Differential diagnosis of this disorder is necessary in cases manifesting diencephalon symptoms or involving lesions bordering the third ventricle without evidence of previous optic neuritis or myelitis.