We present the case an 8-yr-old boy evaluated for anastomotic stenosis of the right pulmonary artery after surgical repair of hemitruncus at 6 wk of age. Pulmonary angiography revealed only mild narrowing and a 10-mm pressure gradient across the anastomosis, but quantitative perfusion imaging demonstrated that the right lung only received 16% of pulmonary blood flow. Subsequently, balloon angioplasty of the anastomotic site was performed, resulting in complete resolution of the stenosis and gradient. Early postangioplasty perfusion imaging demonstrated increased perfusion of the right lung to 35% of total pulmonary blood flow. It is theorized that initially a chronically hyperperfused lung may develop more capacious vessels and recruit new capillaries during the years of hyperperfusion such that a "perfect" angioplasty may result in less than symmetric perfusion. The inexpensive, noninvasive quantitative perfusion study is more sensitive and accurate in evaluating acquired (postsurgical) pulmonary artery stenoses.