The echocardiographically measured maximal rate of left ventricular (LV) wall-thinning in early diastole has been proposed as a new index of diastolic function. However, its causal relationship to transmitral filling velocity (Doppler E-waves) has not been established. We derived the ventricular wall-thinning to filling-velocity relation, using the constant volume attribute of the 4-chamber heart. Left atrio-ventricular geometry was modeled as a constant volume cylinder having a semi-ellipsoid apex and constant external surface. For validation E-wave and color M-Mode images of LV wall thinning were compared to the model predicted filling-velocity to wall-thinning relation. Excellent agreement was observed. We conclude that reasonable assumptions of LV geometry and conservation of volume establish the LV wall-thinning to filling-velocity (Doppler E-wave) relation. Hence, a global, wall-thinning derived diastolic function index is derivable from the transmitral flow velocity (the Doppler E-wave).