Moyamoya disease is a unique chronic progressive cerebrovascular disease characterized by bilateral stenosis or occlusion of the arteries around the circle of Willis with prominent arterial collateral circulation. It can be primary or secondary to genetic syndromes such as Down syndrome. We report a seven year-old girl with a Down syndrome that presented with a disturbance of consciousness, seizures and a right hemiparesia at the age of five. Magnetic resonance imaging showed old cortical ischemic lesions in both cerebral hemispheres and a recent infarction in the territory of the left middle cerebral artery. Brain angiography showed a proximal stenosis of both medial cerebral arteries and a net of collateral vessels, consistent with the diagnosis of moyamoya syndrome. The patient had also an antithrombin III deficiency. Aspirin was indicated and a surgical correction was recommended. However, prior to the procedure, the patient had a new infarction in the territory of the right middle cerebral artery, which caused a severe disability.