Encephalo-duro-arterio-synangiosis (EDAS) is a new surgical operation for childhood moyamoya disease, and its effects have been studied by comparing pre- and postoperative angiograms in 27 patients. The development of collaterals from the external carotid arterial system into the territory of the middle cerebral artery was excellent in 16 of 54 cerebral hemispheres after EDAS, good in 25, and poor in 13. The development of collaterals after EDAS increased as the stenotic process in the internal carotid artery on preoperative angiograms increased except in the most advanced cases, where it seemed to decrease in comparison with the group with middle grade stenosis. After EDAS, not only the superficial temporal artery, but also the adjacent middle meningeal artery participated in forming collateral pathways. Furthermore, decrease of abnormal net-like vessels was observed when there was good to excellent development of collateral vessels. Stenotic lesions in the internal carotid and posterior cerebral arteries were often seen to progress, indicative of the rapidly progressive nature of childhood moyamoya disease. These results appear to suggest that EDAS should be performed as early as possible in childhood moyamoya disease before the occurrence of an irreversible ischaemic state and/or permanent neurological defects.