OBJECTIVE To study the effect of microsurgery on incranial aneurysm and factors influential in prognosis. METHODS A retrospective analysis was made on the clinical data of 1,041 patients, 505 males and 536 females, aged 44.2 +/- 12.6, with incranial aneurysm treated by microsurgery 1988 approximately 2001, including clipping of aneurysm (982 cases, 94.3%), isolating of the aneurysm (32 cases, 3.1%), and coating of the aneurysm (27 cases, 2.6%). RESULTS 93 patients (8.9%) had multiple aneurysms. 357 aneurysms (31.2%) were in the posterior communicating artery, 261 (22.8%) in the anterior communicating artery, 210 (18.4%) in the internal carotid artery, 158 (13.8%) in the middle cerebral artery, 63 (5.5%) in the anterior cerebral artery, 59 (5.2%) in the vertebral-basilar artery, and 35 (3.1%) in the posterior cerebral artery. The overall operative mortality was 1.8%. Angiography performed post-operationally showed that the aneurysm was occluded in 978 patients (99.6%). The prognosis was related to the age of patient (P = 0.001), Hunt and Hess grading before operation (P = 0.001), diameter of aneurysm (P = 0.000), and whether the aneurysm ruptures during operation (P = 0.006), and was not related to the gender of patient (P = 0.059). CONCLUSION Clipping of the aneurysm is considered the best treatment. Microinvasive neurosurgery has improved significantly the effect of treatment of intracranial aneurysm. Factors influencing prognosis after operation include the age of patient, Hunt and Hess grading before operation, diameter of aneurysm, and whether the aneurysm ruptures during operation.