Moya-Moya disease has been associated to a number of disease entities including Down syndrome. Initial clinical manifestations in our patient were alternating lateralizing seizures with sudden onset hemiparesis. Ct scan demonstrated several infarcts in different stages of evolution, in the territory of left middle cerebral artery. Single digital subtraction angiography showed bilateral occlusion, predominantly on the left side of the supraclinoid portion of the interna carotid arteries with formation of collateral circulation in the diencephalic territory. The association of Moya-Moya disease and Down syndrome is not fortuitous and it is probably due to a congenital vascular dysplasia. It should be suspected in children with trisomy 21 (with or without congenital heart disease) who presents with alternating hemiplegia and convulsions or acute hemiplegia.