Moyamoya disease is a cerebrovascular occlusive disease characterized by progressive stenosis or by occlusion at the terminal portion of the bilateral internal carotid arteries. The unusual vascular network (moyamoya vessels) at the base of the brain with this disease as collateral channels is developed in this disease. Social independence because of cognitive impairment has recently been recognized as an important unsolved social issue with adult moyamoya disease. The patients with cognitive impairment have difficulty in proving their status because the standard neuroradiological and neuropsychological methods to define cognitive impairment with moyamoya disease are not determined. These patients with cognitive impairment should be supported by social welfare as psychologically handicapped persons. Thus Cognitive Dysfunction Survey of the Japanese Patients with Moyamoya Disease (COSMO-JAPAN study) is planned. In this study, we want to establish a standard finding of the cognitive impairment in patients with moyamoya disease.