Chylothorax is a rare but potentially serious complication of cardiac operations. We report here a 72-year-old man who underwent replacement of a descending aneurysm with a synthetic graft for dissecting aneurysm (IIIa). A persistent postoperative chylothorax developed, which necessitated continuous drainage, despite conservative treatment more than 12 days. Thoracoscopic high-frequency ultrasonic coagulation of the thoracic duct without clipping finally stopped chyle production. This method may be useful from the standpoint of minimal access, rapid recovery, less pain, and a shorter operation.