The association between moyamoya phenomena and Down syndrome (DS) is reported in the literature. This paper reports a case of DS, which at age 9 presented right hemiparesis, secondary to the occlusion of the left internal carotid artery; cerebral angiography (CAG) showed a collateral circulation that mimicks the moyamoya phenomenon. Clinical recovery was almost complete; a second CAG after 15 months showed a persistent occlusion of the left internal carotid artery and an opacification of the left middle cerebral artery from abnormal vessels; but the collateral circulation is not enhanced. This case proves that in DS cerebrovascular occlusions may present moyamoya-like phenomena. These differ however from the true moyamoya disease in a number of aspects: the arterial occlusion is unilateral, the evolution is favorable and revascularization does not occur through the peculiar abnormal vessels of the moyamoya syndrome.