Background: The phenotype of individuals with hypertrophic cardiomyopathy (HCM) who exercise regularly is unknown. This study characterized the clinical profile of young athletes with HCM. Methods and Results: The electrical, structural, and functional cardiac parameters from 106 young (14.35 years) athletes with HCM were compared with 101 sedentary HCM patients. A subset of athletes with HCM exhibiting morphologically mild (13.16 mm), concentric disease was compared with 55 healthy athletes with mild physiological left ventricular hypertrophy (LVH). Most athletes with HCM (96%) exhibited T-wave inversion and had milder LVH (15.8±3.4 mm versus 19.7±6.5 mm, P<0.001), larger left ventricular cavity dimensions (47.8±6.0 mm versus 44.3±7.7 mm, P<0.001), and superior indices of diastolic function (average E/E' 7.9±2.4 versus 10.7±3.9, P<0.001) compared with sedentary HCM patients. In athletes with HCM, LVH was frequently (36%) confined to the apex and only 15 individuals (14%) exhibited mild concentric LVH mimicking physiological LVH. In these 15 athletes, conventional structural and functional cardiac parameters showed modest sensitivity and specificity for differentiating HCM from physiological LVH: 13% had a left ventricular cavity >54 mm, 87% had a left atrium.40, and 100% had an E/E' <12. Conclusions: Athletes with HCM exhibit less LVH, larger left ventricular cavities, and normal indices of diastolic function compared with sedentary patients. Only a minority of athletes with HCM constitute the conventional gray zone of mild, concentric LVH. In this minority, conventional echocardiographic parameters alone are insufficient to differentiate HCM from physiological LVH and should be complemented by additional structural and functional assessments to minimize the risk of false reassurance.