According to the diagnostic criteria currently in use, the so-called "unilateral" forms of the moyamoya disease, or rather those in which the disorders of the disease itself--such as occlusion of the internal supraclinoid carotid artery or its terminal branches and development of abnormal collateral circulation in the region of the basal ganglia--are only found in one hemisphere and should be considered as "probable" forms of the disease with a much lower incidence than the bilateral or "defined" forms of the same. One patient with a primary intraventricular hemorrhage (PIVH) in whom an occlusion of the right internal carotid artery was angiographically demonstrated and in whom collateral type moyamoya circulation was found as was the presence of an aneurysm in the right coroid territory is presented. The association of PIVH, unilateral moyamoya disease and aneurysm is infrequent in the literature, with the origin of the bleeding, in some cases, having been attributed to rupture of the aneurysm. Since angiographic control was not available in the patient presented it cannot be excluded that the aneurysm was really a pseudoaneurysm therefore being a consequence, rather than a cause, of the arterial rupture. Thus one of the other mechanisms proposed must be invoked to explain the pathogenesis of the PIVH: rupture of a perforating artery or of a microaneurysm located in the subependimary periventricular region. The treatment recommended for these case of PIVH associated to aneurysm is chirurgical if persistence is demonstrated in successive arteriographies.