Previous reports have discussed incidental disease found on brain magnetic resonance imaging (MRI) scans that had been requested for an unrelated clinical concern or symptom, resulting in a selection bias for disease. However, the prevalence of unexpected abnormalities has not been studied in a healthy population. To evaluate the prevalence of incidental findings on brain MRI scans obtained for a healthy, asymptomatic population without selection bias. Retrospective analysis of brain MRI scans obtained between May 17, 1996, and July 25, 1997, from 1000 volunteers who participated as control subjects for various research protocols at the National Institutes of Health. All participants (age range, 3-83 years; 54.6% male) were determined to be healthy and asymptomatic by physician examination and participant history. Prevalence of abnormalities on brain MRI by category of finding (no referral necessary, routine referral, urgent referral [within 1 week of study], and immediate referral [within 1 to several days of study]). Eighty-two percent of the MRI results were normal. Of the 18% demonstrating incidental abnormal findings, 15.1% required no referral; 1.8%, routine referral; 1.1%, urgent referral; and 0%, immediate referral. In subjects grouped for urgent referral, 2 confirmed primary brain tumors (and a possible but unconfirmed third) were found, demonstrating a prevalence of at least 0.2%. Asymptomatic subjects present with a variety of abnormalities, providing valuable information on disease prevalence in a presumed healthy population. A small percentage of these findings require urgent medical attention and/or additional studies.