The most important task for an effective way of SAH prevention is to estimate the rupture risk of unruptured intracranial aneurysms (UIAs) and to reduce the operative risk for clipping. A multi-center study on the treatment of UIAs was organized in 1999, supported by the Japanese Ministry of Health and Welfare. In five Japanese National Hospitals, all UIAs were registered and analyzed. In this study, we investigated the management outcome in 146 patients with asymptomatic incidentally discovered aneurysms to evaluate the benefit of preventive surgery and conservative treatment. Ninety-seven patients underwent surgery and three patients (3.1%) became moderately or severely disabled after surgery. There was no mortality after surgery or endovascular therapy. Among the 39 patients who underwent conservative therapy, four (10.3%) suffered from subsequent aneurysm rupture. Radical treatment should be considered for the patients with incidental unruptured aneurysms.