In the clinical evaluation of left ventricular (LV) function in humans, there has been growing interest in the diagnostic and prognostic power of measurements or derived indexes of LV systolic relaxation, ie, LV isovolumetric pressure fall and early rapid filling. Most of these measurements have been appreciated as excellent predictors of LV systolic function, in particular in the early phases of ischemic and hypertrophic cardiomyopathies. Misinterpretation of such measurements, however, has often contributed to the many controversies concerning the diagnosis of diastolic cardiac failure. To most clinicians, the concept and diagnosis of LV diastolic dysfunction or failure are not easy. Part of this problem relates to the fact that many, mainly clinical, investigators erroneously persist in applying different definitions of diastole, mostly on empirical-historical grounds, depending on whether they consider the heart as a pump instead of a “muscular” pump or analyze cardiac hemodynamics as a function of time or as pressure-volume (P-V) relations. Meanwhile, diastolic failure of the heart has become a widely recognized clinical entity. In a recent review, we identified diastolic failure as a condition resulting from an increased resistance to ventricular filling and leading to symptoms of congestion due to an inappropriate shift of the diastolic P-V relation.1 Causes of diastolic failure, and hence of shifts in the diastolic P-V relation, are inappropriate tachycardia, decreased diastolic myocardial/ventricular compliance, and impaired LV systolic relaxation, ie, impaired LV isovolumetric pressure fall or early rapid filling. Further, in this issue of Circulation , Ishizaka and colleagues2 have again drawn attention to the importance of impaired LV systolic relaxation as an early event in experimental heart failure. These investigators examined the contribution of changes in end-systolic volume and of the loading sequence to impaired LV isovolumetric relaxation in overdrive-pacing heart failure. They used caval occlusion to decrease end-diastolic …