The autopsy files and preparations of unruptured incidental intracranial aneurysms seen at the Montefiore Medical Center between 1951 and 1987 were reviewed. There were 84 patients with 102 unruptured aneurysms in a total of 10,259 autopsies, giving a prevalence of 0.8%. Sixteen of the 84 (19%) had multiple aneurysms. The thickness of walls of aneurysms could be estimated in 78 of 102 aneurysms, and was determined to be either thin or thin and thick in 71 aneurysms. In this study, four noteworthy factors were found: (1) the incidence of unruptured aneurysms was higher in elderly patients aged 60 years or older, and the peak percentage was 1.2% in the seventh decade; (2) aneurysms occurred more frequently in females than males, with a ratio of 53:31; (3) the most common site of aneurysms was the middle cerebral artery; 37 of 102 aneurysms (36%) occurred on it; and (4) the rate of small aneurysms was very high; 50 of 93 aneurysms (54%) were 4 mm or less in diameter, and 33 aneurysms (35%) were 5-9 mm in diameter. However, relationships could not be found between age distribution and location, size, or thickness of walls; between gender and size or thickness of walls; between location and size or thickness of walls; or between size and thickness of walls. Based on published statistics on subarachnoid hemorrhage and this study, the rupture rate of unruptured aneurysms seems to be very low. Although the risk of rupture may be relatively low in small aneurysms, its low risk probably cannot be explained adequately by morphological examination only.