Background The incidence of coarctation after stage I Norwood procedure varies between 11% and 37%, and it contributes to late death after this operation. We describe the incidence and report the results of percutaneous balloon angioplasty of neoaortic arch obstruction in patients after the modified Norwood procedure for hypoplastic left heart syndrome (HLHS). Methods During a period of 5 years, 136 patients (94 male, 42 female) underwent a first stage modified Norwood procedure for HLHS. All 95 survivors (69.8%) underwent cardiac catheterization before the second stage. Neoaortic arch obstruction was diagnosed on documentation of a peak systolic gradient of >10 mm Hg across the arch associated with angiographic evidence of localized narrowing of the aortic lumen. Results Twenty-one (22.1%) of the 95 survivors were documented to have neoaortic arch obstruction. Seventeen patients underwent percutaneous balloon angioplasty for the relief of the neoaortic arch obstruction. The predilatation peak gradient across the arch was reduced significantly by angioplasty from 28.6 ± 16.9 mm Hg (range 10 to 73 mm Hg) to 5.3 ± 5.2 mm Hg (range 0 to 19 mm Hg) (P <.001). A final gradient <10 mm Hg or <70% of the starting gradient was considered a success. Conclusion The absence of serious sequelae after the procedure or need for reintervention, as shown by our study, makes balloon angioplasty an effective technique and the treatment of choice for the relief of recoarctation of the neoaortic arch in patients with staged palliation of HLHS. (Am Heart J 2000;140:227-31.)