We report the case of a boy with a gigantic aneurysm in the thoraco-abdominal region which was detected by a chest X-ray taken prior to surgical correction of ptosis of the eyelids at 11 months of age. At 18 months, he successfully underwent aneurysm exclusion and bypass grafting. A biopsy from the thoracic aorta revealed medial degeneration with conspicuous smooth muscle cell involvement. Laboratory examination showed altered elastase activity in the granulocytes and whole blood. The present case may represent a unique form of aneurysm in infancy.