BACKGROUND Post-operative chylothorax in patients with congenital heart disease is a challenging problem with substantial morbidity and mortality. Currently, the etiology of chylothorax is poorly understood and treatment options are limited. OBJECTIVES This study aimed to report lymphatic imaging findings, determine the mechanism of chylothorax after cardiac surgery, and analyze the outcomes of lymphatic embolization. METHODS We conducted a retrospective review of 25 patients with congenital heart disease and post-operative chylothorax who presented for lymphatic imaging and intervention between July 2012 and August 2016. RESULTS Based on dynamic contrast-enhanced magnetic resonance lymphangiography and intranodal lymphangiography, we identified 3 distinct etiologies of chylothorax: 2 patients (8%) with traumatic leak from a thoracic duct (TD) branch, 14 patients (56%) with pulmonary lymphatic perfusion syndrome (PLPS), and 9 patients (36%) with central lymphatic flow disorder (CLFD), the latter defined as abnormal central lymphatic flow, effusions in more than 1 compartment, and dermal backflow. Patients with traumatic leak and PLPS were combined into 1 group of 16 patients without CLFD, of whom 14 (88%) had an intact TD. Sixteen patients underwent lymphatic intervention, including complete TD embolization. All 16 patients had resolution of chylothorax, with a median of 7.5 days from intervention to chest tube removal and 15 days from intervention to discharge. The 9 patients with CLFD were considered a separate group, of whom 3 (33%) had an intact TD. Seven patients underwent lymphatic intervention but none survived. CONCLUSIONS Most patients in this study had nontraumatic chylothorax and dynamic contrast-enhanced magnetic resonance lymphangiography was essential to determine etiology. Lymphatic embolization was successful in patients with traumatic leak and PLPS and, thus, should be considered first-line treatment. Interventions in patients with CLFD were not successful to resolve chylothorax and alternate approaches need to be developed. (C) 2017 by the American College of Cardiology Foundation.