Thoracic duct injury is a well described entity as complication from cardiothoracic surgery. However tension chylothorax is rare, and may become a life-threating condition. Objective: To present 2 pediatric patients who developed hemodynamic and respiratory failures secondary to chylothorax. Patients: The first patient was a 2-month-old boy who developed chylothorax three weeks after a Norwood-Sano surgery; he showed a severe respiratory and hemodynamic collapse. The second patient was a one-month old baby who developed an acute respiratory failure and oliguria two days after a patent ductus arteriosus surgery. In both cases the chest tube placement resulted in the release of chyle under pressure and resolution of the symptoms. Conclusions: These two cases demostrate how chylothorax may provoke severe hemodynamic and respiratory effects. Early recognition and treatment of this condition is important for improved outcome.