Although well described in pediatric populations, series of congenital unicuspid aortic valves in adults are limited and mainly arise from surgical reports. Also, the incidence of this rare congenital anomaly has never been estimated in an adult echocardiographic population. Between January 1990 and May 2002, the authors' echocardiographic database was systematically reviewed to identify adult patients with congenital unicuspid aortic valve. Echocardiographic studies were reviewed off-line to define morphologic characteristics and evaluate ascending aortic dimensions. Twenty-one adult patients with congenital unicuspid aortic valve were identified, resulting in an estimated incidence in the referral population of approximately 0.02%. Aortic stenosis was the predominant associated hemodynamic lesion; on average, this required surgical intervention during the third decade of life. All valves were unicommissural with a posteriorly positioned commissural attachment. Of particular clinical importance, coexisting ascending aortic dilatation was found in almost half of the cohort (48%), despite the younger patient age. Adult congenital unicuspid aortic valve is a rare cardiac anomaly, even in a large referral population. On average, patients will present for cardiac surgery in their third decade of life. Aortic dilatation in association with unicuspid aortic valves should be well recognized and routinely assessed in the initial evaluation and follow up of these patients.