A study was conducted to evaluate the postoperative complications associated with incidental asymptomatic aneurysms in 30 patients with 34 cerebral aneurysms who suffered from ischemic or hemorrhagic cerebrovascular diseases. On postoperative CT scan, seven patients were shown to have developed new infarctions and seven patients had new cerebral or cerebellar hemorrhages. Among the patients who developed new infarctions, the problem was directly due to operative procedures in three cases, and in another two cases the problem was thought to be associated with STA-MCA anastomosis which had been simultaneously performed. The other two cases were considered to be due to hemodynamic ischemia during surgery and anesthesia. Three of the patients with infarction almost totally recovered to their preoperative neurological state.Two of the seven patients with hemorrhage had received preoperative antiplatelet therapy for their original diseases, and one had liver cirrhosis with thrombocytopenia. One received STA-MCA anastomosis as mentioned above and both hemorrhage and infarction developed. Two patients died of late systemic complications.Aside from infarctions confirmed by CT scan, five patients had postoperative focal neurological deterioration, from which they made an almost total recovery. These cases may have been due to transient ischemia. Nine patients suffered postoperative epileptic seizures. On CT scan, one was shown to have multiple cerebral and cerebellar hemorrhages, two to have infarctions, and one to have both. Five patients had no remarkable signs on CT scan except for slight cerebral edema. Five developed status epilepticus, and one died of diffuse cerebral edema secondary to uncontrollable status epilepticus. The other two patients died of late systemic complications after developing an impaired mental state after seizures. There was no correlation between these complications and the patients' age or length of time between the first onset of cerebrovascular disease and surgery for aneurysms. Of the 30 patients, the surgical morbidity rate was 70%, including transient neurological deficits and epilepsy, and the late mortality rate was 13% (four patients), including systemic complications.In conclusion, patients who are found to have aneurysms incidentally during a course of ischemic or hemorrhagic cerebrovascular disease have a high risk in direct surgery, and intensive care should be taken during surgery and in the pre- and postoperative period. Preoperative anticonvulsants, evaluation of coagulation capacity, stable blood pressure during surgery and the postoperative period, and no non-essential procedures other than aneurysm surgery, are important considerations for these patients.