We used the Mayo Clinic medical records linkage system to detect all cases of intracranial vascular malformation (IVM) among residents of Olmsted County, Minnesota from 1965 to 1992 and found 48 IVMs over the 27-year period, including four patients with IVM detected at autopsy, without prior symptoms. Twenty-nine of 48 patients were symptomatic at presentation. From 1985 to 1992, the age- and sex-adjusted detection rate for IVMs excluding asymptomatic autopsy cases was 2.75 in 100,000 person-years (P-Y) (95% CI, 1.6 to 3.9). From 1965 to 1992, the detection rate was 1.84 in 100,000 P-Y (95% CI, 1.3 to 2.4). On January 1, 1990, the age- and sex-adjusted prevalence rate of identified IVMs was 19.0 in 100,000 P-Y (95% CI, 10.5 to 27.5). Detection rates increased from 1965 to 1992, probably due to increased identification of cases because of advances in imaging technology. This study provides unique data on incidence rates of detection and prevalence of IVMs on a population basis.