Chylothorax following an intrapericardial cardiac operation is rare, and we are aware of only 12 reported cases. Nine followed median sternotomy for treatment of congenital heart disorders or acquired valvular disease; more than expected were reoperations (23%). This report documents that this complication may also occur following myocardial revascularization with internal mammary graft and describes the anatomy that makes this possible. This rare complication is important because of the high morbidity of prolonged tube drainage and the fact that nearly half of the affected patients underwent reoperation (5 of 13). Dilated lymphatics exuding chyle have been found at operations undertaken to control lymph fistulas and were located in anterior thymic tissue previously divided by electrocautery. Awareness of the significance of lymph encountered during cardiac operations, particularly during reoperation and near the origin of the internal mammary artery, may alert the surgeon that the stage is set for a disabling complication. Electrocautery may be an unreliable means of control as lymph contains little coagulable material; suture is recommended instead.