CASE PRESENTATION: Pre-transplant, he required oxygen (O2) at 2 liters per minute (lpm) to maintain saturations (SaO2) ≥ 88%. However, post-transplantation, his oxygen requirements increased to 10 lpm at rest and up to 25 lpm during upright activity to maintain SaO2 ≥ 88%. Computed tomography angiogram of the chest was negative for pulmonary embolism, arteriovenous malformations or a parenchymal process. Contrast enhanced echocardiography showed both right to left intra-cardiac and trans-pulmonary shunting. However, right heart catheterization noted normal pressures in the RA, right ventricle and pulmonary artery. At 1 month post-transplant, he remained severely hypoxemic. The patient was treated with intra-venous methylene blue with transient improvement in O2 requirements. At 4 months post-transplant, reassessment of the patients’ shunt fraction showed significant progression from 11% pre-operatively to 28%. Trans-esophageal echocardiography again showed the presence of right to left intra-cardiac shunting. The PFO was closed percutaneously using an Amplatzer closure device. The patient’s oxygenation showed immediate improvement, reducing resting O2 from 10 to 4 lpm and from 25 to 10 lpm during upright activity.