Surgical treatment of unruptured aneurysms is gaining increased support owing to the recently defined poor long-term natural history of these aneurysms. The benefit of treatment ultimately depends on the relative risk of subsequent aneurysm rupture in untreated patients versus the risk of surgery. To identify those patients at a higher risk from surgery, the authors reviewed the management of 172 patients with unruptured intracranial aneurysms treated at their institution. The size of the aneurysms ranged from 3 to 45 mm (mean 13.7 mm). Twenty-two patients (12.8%) had aneurysms in the posterior circulation, and 32 (18.6%) of these were giant aneurysms. Major morbidity occurred in 12 patients (6.9%) and five patients (2.9%) died. Multivariate logistic analysis of several risk factors revealed that aneurysm size and location had an independent correlation with surgical outcome and that patient age approached statistical significance. Patients presenting with ischemic cerebrovascular disease, in particular, did not have a higher risk of a poor outcome. A simple classification for predicting patients at high risk from surgical morbidity and mortality is proposed. Preoperative grading is based on the size and location of the aneurysm and patient's age. The lowest grade is given to young patients with small anterior circulation aneurysms, and the highest grade includes elderly patients with complex giant posterior circulation aneurysms. A retrospective analysis of this classification demonstrated a strong correlation with postoperative outcome. The incidence of poor outcome progressively increased with a higher grade, ranging from 0% in Grade 0 to 66.6% in Grade VI. An analysis of this classification on 50 consecutive surgically treated patients with unruptured aneurysms not included in the analysis also validated the predictive value of this system. Along with predicting outcome, this classification should provide a standardized format for comparison of results from different clinical centers as well as different therapeutic techniques (surgical vs. endovascular) without omission of significant risk factors found to influence outcome.