We performed examinations and echocardiographic studies in 35 patients with Down syndrome (aged 20 +/- 4.2 years) with no known intracardiac disease. Sixteen patients (46%) had mitral valve prolapse; two of these also had tricuspid valve prolapse. Two had aortic regurgitation. Valve regurgitation was present in 4 (17%) of 23 patients more than 18 years of age but in none of the 12 patients 18 years of age or younger. We recommend screening of adolescent and young adult patients with Down syndrome for the development of valve dysfunction, especially before dental or surgical procedures.