In this work, we present a systematic phantom comparison and clinical application of noninvasive pressure difference mapping in the human aorta based on time-resolved 3D phase contrast data. Relative pressure differences were calculated based on integration and iterative refinement of pressure gradients derived from MR-based three-directional velocity vector fields (flow-sensitive 4D MRI with spatial/temporal resolution ∼ 2.1 mm 3/40 ms) using the Navier-Stokes equation. After in vitro study using a stenosis phantom, time-resolved 3D pressure gradients were systematically evaluated in the thoracic aorta in a group of 12 healthy subjects and 6 patients after repair for aortic coarctation. Results from the phantom study showed good agreement with expected values and standard methods (Bernoulli). Data of healthy subjects showed good intersubject consistency and good agreement with the literature. In patients, pressure waveforms showed elevated peak values. Pressure gradients across the stenosis were compared with reference measurements from Doppler ultrasound. The MRI findings demonstrated a significant correlation (r = 0.96, P < 0.05) but moderate underestimation (14.7% ± 15.5%) compared with ultrasound when the maximum pressure difference for all possible paths connecting proximal and distal locations of the stenosis were used. This study demonstrates the potential of the applied approach to derive additional quantitative information such as pressure gradients from time-resolved 3D phase contrast MRI.