This study investigated the true incidence and risk factors for prolonged pleural effusion/chylothorax (PPE/C) following pediatric cardiac surgery, as well as its impact on other postoperative outcomes. This UK-based study collected data prospectively from five centers between 2015 and 2017, analyzing a total of 3,090 procedures (surgical and hybrid). This study found an incidence of 6.5 percent for PPE/C, highest after Fontan and other complex cases, occurring at median postoperative day six. Interestingly, cases of PPE/C with no other morbidity were associated with an eight-day increase in hospital length of stay (LOS), compared to those with multimorbidity, where the LOS was not significantly affected, but early mortality was higher.