We report the results of a statistical analysis of the long-term outcome of 54 patients with 72 unruptured cerebral aneurysms, and identify the factors for predicting subsequent ruptures. Twenty-two patients died during the observation period, which averaged 43.7 months. The 5-year survival rate was 56%. Aneurysms ruptured in 11 patients (20.4%), 10 of whom died without undergoing surgery. The annual bleeding rate was 1.92%. The average size of the 11 ruptured aneurysms was 13.1 mm. In 4 patients, however, bleeding occurred in unruptured cerebral aneurysms of 4 and 5 mm, which suggests that leaving unruptured cerebral aneurysms of less than 10 mm in size untreated is hazardous. According to the Cox proportional hazards model, the shape and location of the aneurysm and the presence of hypertension were the most important factors for predicting a subsequent rupture. Our data suggest that unruptured cerebral aneurysms arising from the vertebrobasilar and middle cerebral arteries of 10-19 mm size and of multilobes had a statistically high probability of subsequent bleeding. Although 20 patients with 28 unruptured cerebral aneurysms were followed through repeated examinations, we could not correlate the risk of subsequent bleeding with changes in the size of the aneurysm.