An index of filling efficiency incorporating stiffness and relaxation (S&R) parameters has not been derived or validated although numerous studies have focused on the effects of altered relaxation or stiffness on early rapid filling and diastolic function. Previous studies show that S&R parameters can be obtained from early rapid filling (Doppler E-wave) via kinematic modeling. E-wave contours are governed by harmonic oscillatory motion, modeled via the parameterized diastolic filling (PDF) formalism. The previously validated model determines three (unique) oscillator parameters from each E-wave having established physiologic analogues: x o (load), c (relaxation/viscoelasticity), and k (chamber stiffness). We define the dimensionless, filling-volume based kinematic filling efficiency index (KFEI) as the ratio of the velocity-time integral (VTI) of the actual clinical E-wave contour fit via PDF to the VTI of the PDF model-predicted ideal E-wave contour having the same x o and k, but with no resistance to filling (c=0). To validate the new index, Doppler E-waves from 36 subjects with normal ventricular function, 17 diabetic and 19 well-matched non-diabetic controls, were analyzed. E-wave parameters x o , c, and k and KFEI were computed for each subject and compared. In concordance with prior human and animal studies in which c differentiated between normal and diabetic hearts, KFEI differentiated (p<0.001) between non-diabetics (55.8±3.3%) and diabetics (49.1±3.3%). Thus, the new index introduces and validates the concept of filling efficiency, and using diabetes as a working example, provides quantitative and mechanistic insight into how S&R affects ventricular filling efficiency.