Arterial aneurysms are very rare in children, and aneurysms with an idiopathic etiology have been reported in only a few cases. In most cases, aneurysms are caused by infection, arteritides, collagen vascular disease, vascular malformations, or trauma. We report the case of an 11-year-old girl with an idiopathic and symptomatic left common iliac artery aneurysm. The aneurysm was resected and replaced by a reversed femoral vein. Because the external iliac artery was atrophied, an additional bypass with a reversed great saphenous vein was made. In the postoperative work-up, no etiologic cause could be found for the development of the aneurysm. The symptoms immediately subsided after the operation.