Functional angiographic investigation and percutaneous embolization using detachable balloons in nine carotid cavernous aneurysms, three petrous aneurysms, one vertebral artery aneurysm, and one posterior inferior cerebellar aneurysm are reported. A double-lumen balloon catheter is used to evaluate acute tolerance to occlusion of the carotid or vertebral arteries. Occlusion is tested under systemic heparinization. Local perfusion of heparinized saline, proximal as well as distal, to the balloon occlusion is used. The procedure was successful in all but one cavernous aneurysm. The arterial lumen was sacrificed in all cases. Clinical improvement occurred in all successful cases. Retro-orbital pain was relieved in all. Ocular cranial nerve palsies improved or resolved in most. One delayed ophthalmic episode that improved represents the only complication. No such embolic problems occurred in any case in which the cavernous carotid artery was occluded by balloon trapping. The delayed embolic complications after carotid artery occlusion are related to the collateral vessels to the C-4 and C-5 segments of the artery. Balloon trapping decreases the length of the thrombosed segment and prevents retrograde filing of the aneurysm.