The effect of encephalo-duro-arterio-synangiosis (EDAS) upon chronic cerebrovascular ischemia in 65 pediatric moyamoya patients was evaluated by the postoperative interval before complete disappearance of ischemic attacks and changes in pre- and postoperative intelligence (IQ) or development quotients (DQ). The ischemic attacks disappeared after a mean period of 239 days, in three-fourths of patients within a year and in about one-fourth within the second year. This was very fast compared with the natural course of the disease. There was no significant difference in DQ/IQ before and after the operation. The mentally normal (IQ/DQ greater than or equal to 86) population in the postoperative patients was greater than in the natural course of the disease, although fewer in the preoperative group. This shows that EDAS delayed or prevented the deterioration in mental capacity usually present but often overlooked in the natural course of pediatric moyamoya disease.