AbstractCardiac tamponade is a known life-threatening complication of extracorporeal membrane oxygenation (ECMO), often secondary to hemopericardium from major vascular or cardiac perforation. We present the unique case of a neonate with a milky pericardial effusion causing tamponade after ECMO cannulation, managed successfully with pericardial window. Understanding ECMO physiology and its effect on the classic presentation of tamponade is critical and can prevent delays in diagnosis. While hemopericardium is most commonly seen in these cases, findings of a non-bloody, milky effusion should prompt further workup for infection, chylopericardium or total parenteral nutrition-associated pericardial effusion, as the appropriate management can mitigate immediate and potential long-term sequelae.