Cerebral intracranial aneurysms are common. In angiographic and autopsy studies, estimates for prevalence vary between 0.2 and 9.9 percent. The methods used to detect the aneurysms markedly influenced the proportion of aneurysms. Retrospective autopsy studies probably give an underestimation and prospective angiography studies an overestimation of the actual prevalence. Accurate data on the prevalence of intracranial aneurysms are essential in evaluating the results of screening programs for aneurysms in general population.