Ascending aortic aneurysm is uncommon in the paediatric population, and because of the rarity the aetiology, natural progression, and prognosis of the disease remain unknown. A 7-year-old boy with ascending thoracic aortic aneurysm (60×67 mm) underwent graft anastomosis from the sinotubular junction to the undersurface of the arch. Analytical determinations including karyotyping and genetic mapping were all normal. To our knowledge, idiopathic aneurysm of the ascending aorta in children is very rare.