Hypertrophic cardiomyopathy (HCM) is a genetic disease occurring in up to 1 in 500 of the general population. HCM is often undiagnosed or misdiagnosed, and asymptomatic cases are often unrecognized. Asymmetric left ventricular hypertrophy (LVH) most often develops during the period of rapid body growth of adolescence, but it may be present in childhood or even before birth. Progressive LVH after age 20 is uncommon, but initial diagnosis even in old age is not. The hypertrophy predominantly involves the left ventricle, and is often more marked than in any other cardiac disease. Hypertrophy may involve the right ventricle, and an atrial myopathy may be progressive (left atrial enlargement and increased risks of atrial fibrillation). Right ventricle and left atrium involvement may be secondary to the left ventricular disease and/or a primary consequence of the basic molecular defect. The LVH represents hypertrophy and hyperplasia of several cell types, including cardiac myocytes, fibroblasts, and smooth muscle cells, along with excessive collagen and matrix deposition, and abnormalities of the microvasculature. The normal parallel arrangement of myocytes is often disturbed (fibre disarray).