Two adult cases of cerebrovascular Moyamoya disease, associated with basilar aneurysm are reported. At the follow-up study of angiogram performed 14 months later in one case, the aneurysmal shadow was slightly larger than before. In the other case, craniotomy was performed and two aneurysms were confirmed to be the saccular aneurysms arising from the basilar head and superior cerebellar artery junction, respectively. From these facts, it is suggested that these aneurysms were not pseudo-aneurysms but sacuular aneurysms. As for the pseudo-aneurysm associated with cerebrovascular Moyamoya disease, we discussed in our previous report. It has been generally through that the mechanism of the occurence of symptoms in adult patient with cerebrovascular Moyamoya disease is due to rupture of abnormal vessels in the subarachnoid space. But from our experiences, it is suggested that there should be another mechanicsm, which begins with rupture of these basilar aneurysms. In addition to that, the growing mechanisms of basilar aneurysm associated with cerebrovascular Moyamoya disease is discussed.