Fusiform aneurysms (FA) and dissecting aneurysms (DA) of the vertebro-basilar systems have been thought to be rare, but recently reports of these aneurysms have increased. The differential diagnosis between FA and DA is requisite for deciding therapy or for prognosis, but it is often difficult to distinguish these aneurysms even if angiographies are conducted. The authors have treated 6 cases with FA and 5 cases with DA during the past 17 years. As the initial symptom, subarachnoid hemorrhage (SAH) was noted in all 6 cases with FA. On the other hand, 2 cases of SAH, 2 cases of brain stem infarction, and 1 case of ischemic attack were noted in DA cases. The aneurysmal locations of FA were at the vertebral artery (VA) in all 6 cases, and those of DA were at the VA in 3 cases and at the basilar artery (BA) in 2 cases. As angiographic findings of DA, pearl & string signs were demonstrated in 3 cases, and retention of contrast medium was noted in 3 cases. Diagnosis of FA is comparatively easy on angiography but when angiospasm exists, it is difficult to differentiate DA from FA. Consequently, repeated angiography is recommended. MR imaging findings of 2 cases with FA were compared with those of 3 cases with DA. No abnormal findings, excepting a dilatation of the signal-void area corresponding to the arterial blood flow were shown in FA, but various abnormalities were detected in all of the 3 cases with DA. Namely, an intimal flap and a double lumen in 1 case, intra-mural hematoma in 1 case, and a hematoma adjacent to the parent artery in 2 cases were demonstrated. Thus, MR imaging was thought to be a useful means for distinguishing between FA and DA.