The evaluation of left ventricular (LV) compliance by use of the pressure-volume (P-V) relationship encounters several serious difficulties. Since the P-V relationship is curvilinear, it is difficult to quantitate. Furthermore, alterations of resting heart size and geometry also produce marked changes in the P-V curve. The first derivative of the P-V relationship, however, is a precisely linear function expressed by the formula dP/dV = aP+b. The slope of this linear function, a, termed the passive elastic modulus, has been shown to be independent of initial volume and primarily and predominantly determined by changes in the stiffness of the myocardium. Myocardial wall stiffness was evaluated in three groups of subjects during LV catheterization. In 13 normal subjects a = 0.005; in 13 with coronary artery disease a = 0.011; and in 12 with acute infarction a = 0.045. The differences in stiffness among the groups were highly significant (P< 0.005).It was concluded that a measurable change in ventricular compliance occurs with the development of coronary artery disease and that a further increase in wall stiffness occurs with the development of acute infarction. The magnitude of increase in LV wall stiffness correlated directly with immediate prognosis: 87% of those subjects with a &Dgr;P/&Dgr;V greater than 0.5 mm Hg/cc died of power failure during the acute stage of their illness. These alterations in compliance may invalidate certain traditional concepts of LV function and heart failure.