Purpose : Aortic valve disease (AVD) in the form of stenosis, insufficiency, or congenital defect will disrupt normal function beyond the valve itself. This includes an increase in cardiac afterload and a drastic alteration in post-valvular 3D blood flow patterns 1, 2. The current AHA/ACC standard-of-care guidelines, however, assess disease severity based on simplified measurements local to the valve, such as: peak velocity, effective orifice area, regurgitation, aortic diameter and transvalvular pressure gradient 3. Paradoxically, it is known that similarly classified AVD patients under these guideline metrics can exhibit radically divergent outcomes — implying an incomplete characterization of the disease 4. For this reason, functional assessment and risk-stratification may benefit from a robust methodology capable of quantifying the energetic load placed on the left ventricle (LV) due to the presence of AVD. The measurement of viscous energy loss, a parameter which is directly responsible for increased cardiac afterload and is independent of pressure recovery effects, is a promising candidate to quantify LV loading. With this in mind, the 4D flow technique (time-resolved 3D phase-contrast MRI with all principal velocity directions encoded) provides the necessary information to calculate this parameter. Therefore, we present a theoretical basis for the use of 4D flow MRI to characterize in-vivo energy loss and apply the technique in a pilot study of patients with aortic valve stenosis (n = 13) or aortic dilation (n = 17) as compared to normal controls (n = 12).