A case of a huge thoracoabdominal aortic aneurysm in an eighteen-month-old boy is reported. Surgical treatment was successfully performed by thromboexclusion of the aneurysm with thoracoabdominal aortic bypass using a low-porosity woven Dacron graft 10 mm in diameter and of sufficient surplus length. During the early postoperative period, he developed moderate hydronephrosis, owing to compression of the left ureter by the graft, but no further deterioration was seen. Follow-up angiographies performed four and six years after surgery revealed straightening of the graft and slight stretching of the aorta at the distal anastomosis, but no stenosis was found. Now, seven and a half years after surgery, he has no pressure gradient between upper and lower extremities.