Objective : There have been no clinical studies regarding the epidemiology and treatment outcome for unruptured intracranial aneurysm (UIA) in South Korea yet. Thus, The Korean Society of Cerebrovascular Surgery (KSCVS) decided to evaluate the clinical and epidemiological characteristics, and outcome of the treatment of UIA in 2006, using the nationwide multicenter survey in South Korea. Methods : A total of 1,696 cases were enrolled retrospectively over one year at 48 hospitals. The following data were obtained from all patients : age, sex, presence of symptoms, location and size of the aneurysm, treatment modality, presence of risk factors for stroke, and the postoperative 3D-day morbidity and mortality. Results : The demographic data showed female predominance and peak age of seventh and sixth decades. Supraclinoid internal carotid artery was the most common site of aneurysms with a mean size of 5.6 mm. Eight-hundred-forty-six patients (49.9%) were treated with clipping, 824 (48.6%) with coiling, and 26 with combined method. The choice of the treatment modalities was related to hospital (p=0.000), age (p=0.000), presence of symptom (p=0.003), and location of aneurysm (p=0.000). The overall 30-day morbidity and mortality were 7.4% and 0.3%, respectively. The 30-day mortality was 0.4% for clipping and 0.2% for coiling, and morbidity was 8.4% for clipping and 6.3% for coiling. Age (p=0.010), presence of symptoms (p=0.034), size (p=0.000) of aneurysm, and diabetes mellitus (p=0.000) were significant prognostic factors, while treatment modality was not. Conclusion : This first nation-wide multicenter survey on UIAs demonstrates the epidemiological and clinical characteristics, outcome and the prognostic factors of the treatment of UIAs in South Korea. The 30-day postoperative outcome for UIAs seems to be reasonable morbidity and mortality in South Korea.