Hypertrophic cardiomyopathy (HCM) is a genetic disease with an autosomal dominant pattern of inheritance which is characterized by left ventricular hypertrophy (LVH). HCM is often caused by missense mutations of genes that encode for sarcomeric proteins. The LVH varies markedly in patients with identical sarcomeric gene mutations, and notably, 20 to 40% of subjects with disease mutation do not have LVH as assessed by echocardiography. These findings suggest that other factors affect LV wall thickness in HCM. We wish (1) to investigate the potential role of IGF-I and its binding protein, IGFBP3, in determining increased LV mass in HCM caused by sarcomeric mutations; and (2) to assess myocardial ultrasound backscatter, exercise tolerance, and propensity to arrhythmias, in subjects who have inherited sarcomeric mutations but who do not have LVH.