It is important to know the characteristics of aneurysms that tend to cause perforation and treatment of these perforations to reduce the morbi/mortality of the endovascular treatment for intracranial aneurysms. Factors leading to aneurysmal perforation were analyzed from the view points of aneurysmal status (ruptured or unruptured), size and direction of aneurysmal dome from the parent artery and treatment of perforation during GDC embolization was discussed in 105 consecutive cases. Perforation occurred in three small aneurysms (less than 3 mm in diameter or depth) where the direction of the dome is the same as that of microcatheter advancement. Perforation occurred when a microcatheter was advanced to counteract catheter recoil caused by coil deployment. Haemorrhage occurred in all cases immediately following microcatheter and coil perforation into the subarachnoid space. In all cases, bleeding was controlled by deploying the coil so that it extended from the subarachnoid space back into the intraaneurysmal cavity. In two cases, surgical clipping was required to treat the incompletely obliterated aneurysm. No additional permanent neurological deficit occurred as a result of any of the three perforations. Special care should be taken during the embolization of small aneurysms (less than 3 mm in minimal diameter) where, owing to the shape of the lesion, or fixation of a microcatheter by the stent strut, the antegrade force of the canulating microcatheter is transmitted directly toward the aneurysm dome.