Seventeen cases of unclippable aneurysms were treated by the endovascular balloon technique. Nine of them involved the anterior circulation, and eight involved the posterior circulation. Eleven of them were treated by parent artery occlusion with detachable balloons. Three were treated by endosaccular balloon embolization, and three cases combined with AVMs were occluded using ethylene vinyl alcohol copolymer (EVAL) including feeding arteries of the AVMs. Embolic complications occurred in one case of an IC bifurcation giant aneurysm treated by parent artery occlusion. Ischemic complications also appeared in two cases of aneurysms treated by endosaccular balloon embolization. In one case, the thrombus in the aneurysm propagated into the parent artery and occluded it later. In another case, the displaced balloon had obliterated the parent artery 6 hours after the embolization. Parent artery occlusion is a safe way to treat internal carotid giant aneurysms. However, endosaccular treatment still has some problems, i.e., 1) maintaining the balloon's position to preserve the parent artery, 2) balloon migration into the clot, 3) rupture of the aneurysm during or after treatment. Our studies indicate that endosaccular balloon embolization is still a high risk procedure and should be used only in selected cases, until new embolic agents, such as detachable coils, become available.