Whether endovascular surgery is able to reduce the mass effects of unruptured aneurysms is still controversial, although some reports have suggested efficacy in cases of internal carotid artery aneurysms with cranial nerve palsy. Here we assessed outcome in a series of cases. Between April 1992 and April 2005, 18 patients with unruptured internal carotid artery aneurysms presenting with cranial nerve palsy were treated by endovascular surgery. The patients were two males and 16 females aged from 19 to 84 (mean 59.6 years). Aneurysms were located in the cavernous portion in 14, at the origin of the ophthalmic artery in one and at the origin of P-com in three. The aneurysms were all embolized using Guglielmi detachable coils, Interlocking detachable coils, Cook's detachable coils or Trufill DSC and detachable Balloons were applied to occlude the proximal parent artery.We analyzed the efficacy of endovascular surgery for such aneurysms retrospectively. The mean aneurysm size was 21.4 mm and the mean follow-up period was 57.7 months. Palsy of II(nd) cranial nerve was evident in three patients, of the III(rd) in eight, of the V(th) and V(th) in one each, and of the VI(th) in nine. Post embolization occlusion was complete in nine patients and neck remnant in the other seven. Regarding complications of endovascular surgery, one case (5.6%) showed TIA after embolization. Overall 11 (46%) cranial nerve symptoms showed complete resolution, eight (33%) showed some improvement, and five (21%) were unchanged. In three cases (12.5%), the symptoms worsened after treatment. The shorter the duration of symptoms was a factor predisposing to resolution of symptoms. In complete resolution cases, the timing of treatment after symptoms appeared and the time of complete resolution were in proportion. These results showed that there is no difference in reduction of mass effects between surgical clipping and endovascular surgery for unruptured internal carotid artery aneurysms.With endovascular surgery, the rapidity of treatment after symptoms is the most important factor for successful results.