The prognosis of 27 patients with moyamoya disease was studied. The ages at onset ranged from 11 months to 4-11/12 years. Follow-up study was performed within 4 years from the onset in 13, 5 to 9 years in 5, and 10 to 15 years in 9. Transient ischemic attacks (TIA) occurred most often during the first four years and decreased thereafter. Intellectual deterioration and neurologic deficits increased with time. Outcome included no sequelae in five (19%), occasional TIA or headache alone in nine (33%), mild intellectual and/or motor impairment in seven (26%), requirement for special school or care by parents or institutions after reaching the teen years in three (11%), continuous 24-hour care in two (7%), and death in one (3%). Poor prognosis was correlated with an early age at onset and hypertension.