The mechanisms of ventricular hypertrophy have been reviewed and the cardiovascular lesions responsible have been listed. These include: volume loads (such as an inflow valve regurgitation and left-to-right shunts); pressure loads (such as semilunar valvular stenosis or systemic hypertension); and certain myocardial diseases such as coronary atherosclerosis and the cardiomyopathies. The factors that stimulate hypertrophy are reviewed and the differences between the various types of hypertrophy are discussed. It is concluded that hypertrophy is essentially an adaptive process and that, under certain circumstances when the hypertrophy is inappropriate in degree or kind, disturbances in cardiac function of a serious nature can result, and such disturbance in function, as for example outflow tract obstruction, can stimulate further hypertrophy. The importance of hypertrophy is underlined by reference to hypertrophic diseases of the myocardium (which are characterized by fixed obstruction to left-or-right ventricular outflow), functional characteristics of subvalvular tissue, the differences between right and left ventricular hypertrophy, the contribution of the ventricular septum to structure and function in a hypertrophied heart and the character of the hypertrophic fibers found in various types of hypertrophy. Pulmonary stenosis of various types is compared and contrasted with aortic stenosis, hypertrophy of the ventricular septum is discussed with reference to hypertrophic cardiomyopathy and Bernheim's syndrome and reference is made to bilateral outflow tract stenoses and intracavitary stenosis. Myocardial lesions that are associated with hypertrophy in the absence of pressure or volume load, such as ischemic heart disease and the cardiomyopathies are compared with pressure load lesions. It is concluded that, in congestive cardiomyopathy, hypertrophy, although appreciable, is masked by the pronounced left ventricular dilatation, and it is suggested that failure to achieve adequate adaptive hypertrophy may be the sause of poor power function, progressive heart failure and limited prognosis. Hypertrophic (obstructive) cardiomyopathy presents an apparently unique disorder of ventricular muscle associated with massive hypertrophy of a particular type, and increased ventricular stiffness due to the massive muscle. The hypertrophy has no useful adaptive function and is often associated with out-flow tract obstruction of the left ventricle. The particular characteristics of the hypertrophic muscle and the way in which disorderly contraction and impaired compliance may result are discussed, as are the possible causes of hypertrophic cardiomyopathy.