Portal venous flow (PVF) seems to be affected by gravity, ie., it depends on the body posture. We validated the effect of gravity on PVF in supine and upright positions using an original multi-posture MRI. We compared maximum PVF, PVF velocity, PVF volume, and cross-sectional area of portal vein between supine and upright positions. The mean PVF velocity, PVF volume, maximum PVF, and cross-sectional area in the upright position were significantly lower than those in the supine position. Gravity reduces PVF velocity and volume, and these differences between postures potentially provide new diagnostic information.