Moyamoya disease is a rare clinical disorder, characterized by chronic, progressive occlusion of supraclinoid internal carotid artery and proximal portions of the anterior and middle cerebral arteries, and consequent development of collateral vessels seen on imaging as puff of smoke appearance. Although the pathophysiology is unclear, various hematologic disorders have been associated with development of the moyamoya syndrome. We report a case of moyamoya syndrome in a 3 yr old girl who presented with recurrent ischemic strokes. The angiography was suggestive of moyamoya syndrome. The serum vitamin B12 level was remarkably low with bone marrow picture of megaloblastic anemia. There was also hyperhomocysteinemia. Moyamoya syndrome has not been previously reported to be associated with vitamin B12 defi ciency. We propose that vitamin B12 defi ciency with hyperhomocysteinemia could result in greater risk for arterial thrombosis and thus caused the moyamoya syndrome in our patient