True aneurysm formation at the site of coarctation repair has been increasingly recognized after synthetic patch aortoplasty. A mathematical model was developed to determine the aortic wall stress profile after coarctation repair with this technique. Methods: A two-dimensional nonlinear mathematical model and a three-dimensional finite element model were developed for different physiologic, geometric, and materials properties variables, which were incorporated into an idealized coarctation repair. Results: The models demonstrated that the major variable affecting stress levels in the aortic wall after coarctation repair was the patch geometry. If the patch was allowed to balloon out, the aortic wall stress increased out of proportion to the increase in aortic diameter because of nonlinear effects. The maximal aortic wall stress concentration occurred opposite the patch. Patch stiffness was also an important variable, with a lower stiffness (subclavian flap repair) leading to a higher aortic wall stress for the same patch geometry as a synthetic patch repair. Inferences: Development of true aneurysms after coarctation repair by synthetic patch aortoplasty is likely to result from excessive aortic wall stress due to patch geometry.