Collagen is a stiff structural material with a high tensile strength. While present in the myocardium in relatively small amounts, collagen is the most abundant structural protein of the heart’s connective tissue network. Removal of less than half of the normal amount of collagen results in a dilated ventricle with increased compliance. The hypertrophic response of the myocardium to an abnormal stress includes an increase in collagen concentration, a thickening of existing fibrillar collagen, and the addition of new collagen at all levels of the matrix. The consequence of this remodeling of the collagen matrix is a stiffer left ventricle with diastolic dysfunction. Thus, myocardial collagen is a major factor in determining the left ventricular diastolic pressure-volume relation.