The paucity of information about giant fusiform intracranial aneurysms prompted this review of 120 surgically treated patients. Twenty-five aneurysms were located in the anterior and 95 in the posterior circulation. Six patients suffered from atherosclerosis and only three others had a known arteriopathy. The remaining 111 patients presented with aneurysms resulting from an unknown arterial disorder; these patients were much younger than those harboring atherosclerotic aneurysms. Mass effect occurred in only 50% of cases and hemorrhage in 20%. Eight aneurysms caused transient ischemic attacks. Hunterian proximal occlusion or trapping were dominant among the treatment methods. In contrast to the management of giant saccular aneurysms, the usual thrombotic occlusion of a giant fusiform aneurysm after proximal parent artery occlusion requires the presence of two collateral circulations to prevent infarction, one for the end vessels and another for the perforating vessels that arise from the aneurysm. Although there was some reliance on the circle of Willis and on collateral vessels manufactured at surgery, the extent of natural leptomeningeal and perforating collateral, thalamic, lenticulostriate, and brainstem vessels was astonishing and formerly unknown to the authors. Good outcome occurred in 76% of patients with aneurysms in the anterior circulation; two of the six cases with poor results included patients who were already hemiplegic. Ninety percent of patients with posterior cerebral aneurysms fared well. Only 67% of patients with basilar or vertebral aneurysms had good outcomes, although more (17%) of these patients were in poor condition preoperatively because of brainstem compression.