As a child grows, limitations to the maximum dilatable stent diameter (MDD) will result in stenosis associated with size mismatch. If an implanted stent can be intentionally fractured along its length, a process called “unzipping,” it may eventually be redilated to adult vessel size. Few studies have addressed how a stent can be unzipped using an ultra-high-pressure balloon (UHB) with the smallest balloon diameter. Eleven commercially available stents, three Liberté stents (LS), six genesis renal stents (GS), and two express vascular SD stents (ES), were tested for in vitro unzipping. In addition, using eight stents, we investigated whether a balloon that had unzipped the stent in vitro would work similarly in the vessel of a pig. Finally, we assessed the histological influence of the unzipped stent on the surrounding tissue. In a bench test, LS, GS, and ES were consistently unzipped by a balloon whose diameter was ≥ 1.5, 2.18, and 1.66 times that of MDD, respectively. In animal experiments, LS, GS, and ES were predictably unzipped with balloons of 1.50, 1.81, and 1.66 times the MDD, respectively. After unzipping, the unzipped strut did not damage the surrounding tissue histologically. Use of a UHB enabled unzipping of the stent with a balloon diameter less than two times the MDD enables implantation of a larger stent in the unzipped small stent by incremental steps.