BACKGROUND: In the palliative pathway of single-ventricle physiology, lymphatic abnormalities on T2-weighted magnetic resonance imaging have been shown after the Glenn operation. It is believed that postsurgical hemodynamic changes contribute to the lymphatic changes.However, little is known about how early these abnormalities occur. Our purpose was to determine if lymphatic abnormalities occur as early as before the Glenn operation. METHODS AND RESULTS: We retrospectively reviewed patients with single-ventricle physiology and a T2-weighted magnetic resonance imaging scan before their Glenn operation (superior cavopulmonary connection) at The Children’s Hospital of Philadelphia from 2012 to 2022. Lymphatic perfusion patterns on T2-magnetic resonance imaging were categorized from type 1 (no supraclavicular T2-signal) to type 4 (supraclavicular, mediastinal, lung parenchymal T2-signal). Types 1 and 2 were considered normal variants. Distribution of lymphatic abnormalities were tabulated, as well as secondary outcomes including chylothorax and mortality. Comparison was done using analysis of variance, Kruskal–Wallis test, and Fisher’s exact test. Seventy-one children were included: 30 with hypoplastic left heart syndrome and 41 with nonhypoplastic left heart syndrome. Lymphatic abnormalities were present before Glenn operation in 21% (type 3) and 20% (type 4), and normal lymphatic perfusion patterns (type 1–2) were seen in 59% of patients. Chylothorax was present in 17% (only types 3 and 4). Pre-Glenn mortality and mortality at any time was significantly increased when having a type 4 lymphatic abnormality compared with types 1 and 2 (P=0.04). CONCLUSIONS: Lymphatic abnormalities can be found on T2-weighted magnetic resonance imaging in children with singleventricle physiology before their Glenn operation. Mortality and chylothorax were more prevalent with advancing grade of lymphatic abnormality.