The role of endovascular therapy for the treatment of giant aneurysms is presently being defined. Results derived from the endovascular treatment of giant aneurysms must be compared to the effectiveness and safety of operative treatment and the natural history of the disease. Most reports on the results of endovascular aneurysm treatment are of patients who have failed operative intervention or in whom operative intervention was not attempted because of their poor medical condition or other factors. Thus, the results of these techniques are from a high-risk subgroup. In a recent series of 19 giant aneurysms treated by a variety of techniques, including coils, balloons, and rapidly solidifying polymers, one death resulted after aneurysm rupture during the procedure (86). However, the major cause of mortality was cardiopulmonary complications within the first 2 weeks after the procedure. At present, it may be appropriate to reserve endovascular techniques for patients with no other reasonable therapeutic option. As experience with these techniques is gained, a comparison must be undertaken in a series of patients clinically equivalent to those in surgical series. Presently, the consensus is that endovascular therapy for giant aneurysms is efficacious for parent-vessel occlusion after balloon test occlusion to assess tolerance to sacrifice. Endosaccular occlusion is most effective if the aneurysm contains little thrombus, as determined by the size of the aneurysm seen on CT or MRI (87), as compared to the angiographic image. Small-necked aneurysms are particularly suited to coil occlusion if the aneurysm can be tightly packed. In wide-necked aneurysms, coil occlusion is possible, although the risk of parent-vessel occlusion is high. We often perform balloon test occlusion of the vessel before placing coils in wide-necked aneurysms. Failure of endovascular therapy after complete angiographic obliteration is based on recanalization or regrowth, resulting from device migration or remodeling at the junction of the device with the inflow tract and aneurysm wall, or by migration of the device into thrombus. The effect of aneurysm remnants after balloon or coil occlusion will be determined by long-term follow-up, as emphasized by Fox et al. (20, 63). Whenever there is an aneurysm remnant, some risk of subsequent hemorrhage exists (66). Further device refinement will enhance the safety and effectiveness of the endovascular treatment of giant aneurysms. The use of combined endovascular and conventional surgical techniques may be an increasingly important option in the treatment of giant aneurysms. Endosaccular packing of an aneurysm with occlusive material may not provide the ability to completely exclude the aneurysm from the circulation, and thus, will not necessarily prevent the process of regrowth. A further limitation of the currently implemented endovascular treatment of aneurysms is that fluoroscopy does not provide detailed information of aneurysm remnants due to the superimposition of occlusive materials, which may necessitate the development of new real-time imaging modalities for interventional procedure, such as intravascular ultrasound and ultrafast-sequence MRI.