Moyamoya disease is a rare, chronic cerebrovascular disorder characterized by progressive stenosis of the arteries composing the circle of Willis. The ischemic type of Moyamoya disease progresses insidiously. To prevent irreversible cerebral damage and psychomotor deterioration, early surgical treatment is considered indispensable. The patient may present with nonspecific symptoms and no specific abnormalities on brain MRI, and might be erroneously suspected as having psychosomatic disorder. The disease must be diagnosed as early as possible. Electroencephalography (EEG) is of little value in the diagnosis of the juvenile type of Moyamoya disease, except for the demonstration of "re-build up" after hyperventilation. Half of the children with Moyamoya disease have been demonstrated to exhibit "re-build up" after hyperventilation. Our patient showed normal background activities, no spike discharges and no slowing during hyperventilation. Nonetheless, we emphasize the appearance of irregular high voltage slow waves de novo after hyperventilation. The findings may be potentially useful for the screening of patients with the juvenile type of Moyamoya disease.