Studies by echocardiography and necropsy show that, with rare exception, all patients with idiopathic hypertrophic subaortic stenosis (or hypertrophic cardiomyopathy) have a ventricular septum that is disproportionately hypertrophied compared with the posterobasal left ventricular free wall. This anatomic defect, genetically transmitted as an autosomal, dominant trait, occurs regardless of the presence or absence of obstruction to left ventricular outflow. Asymmetric septal hypertrophy therefore seems to be an appropriate name for this entity, which is a relatively common disease (the nonobstructive form predominates) that often masquerades as other cardiac disorders. Histologically, the genetic defect is manifested by disordered cardiac architecture characterized by hypertrophied, bizarrely shaped, disorganized myocardial cells. These cells are present in the ventricular septum of all patients with asymmetric septal hypertrophy, and they are distributed widely throughout the left ventricle in symptomatic patients without left ventricular outflow obstruction but are confined largely to the septum in patients with obstruction. Implications of these findings relative to the clinical, therapeutic, and pathophysiologic spectrum of the disease are developed.