OBJECTIVES: We sought to examine the diverse factors associated with aneurysms of the posterior circulation. In addition, the results of conventional craniotomy were compared with those of endovascular treatment. METHODS: One hundred and one patients with posterior circulation aneurysms were selected for study inclusion. The factors that might affect the clinical outcomes were studied , such as the initial Hunt-Hess (H-H) grade, aneurysm location, size of the aneurysm, and therapeutic modalities. In addition, the morbidity and mortality rates were analyzed. The treatment outcomes were evaluated using the Glasgow Outcome Scale (GOS) 6 months after the initial insult. RESULTS: The patient population consisted of 67 women and 34 men, with a mean age of 52 (range 28-81 years). The overall morbidity and mortality rates at 6 months were 13.9% (14/101) and 17.8% (18/101), respectively. Sixty-one operations (60.3%) were performed, and 32 patients were treated with endovascular therapy. Forty-two (85.7%) of the 49 patients that had initial H-H grades of I and II had a better prognosis (GOS more than 4) than those with poor H-H grades (P<0.001). Patients that underwent endovascular treatment had better outcomes than those that had clipping (P=0.032). There was no significant difference in outcome according to the size of the aneurysm, location of the aneurysm, or the age of the patients. CONCLUSIONS: The results of this study showed that the factors affecting the prognosis were the initial HH grade and treatment modality. Considering the very high mortality rate in patients with rebleeding, early management may help improve the prognosis of patients with posterior circulation aneurysms. Endovascular therapy should be considered the primary treatment modality in patients with posterior circulation aneurysms.