TWENTY-THREE CASES OF transitional cavernous aneurysms are presented. Aneurysms of this subgroup (of a total of 118 cases of cavernous sinus aneurysms) arise entirely from within the cavernous sinus but project into the extracavernous intradural subarachnoid space, thus resembling other intracranial aneurysms in their increased risk of rupture. Six aneurysms were small (less than 15 mm), 6 were large (15 to 25 mm), and 11 were giant (more than 25 mm). Thirteen patients (57%) had a subarachnoid hemorrhage, nine patients (39%) had compressive symptoms, and one patient (4%) was asymptomatic. A direct surgical approach was performed successfully in 18 cases (78%), and indirect bypass methods were performed in 5 cases. The overall surgical outcome was excellent in 87% of the patients, with three complications (13%) including one fatality. It is suggested that this subgroup is a different entity from other cavernous or extracavernous aneurysms and should be managed aggressively with direct clipping whenever possible because of the increased risk of subarachnoid hemorrhage. A simplified numerical classification system of clinoidal-region aneurysms of the internal carotid artery (including transitional aneurysms) is also proposed.