Ascending aorta mycotic aneurysm is a rare entity. It is a life-threatening condition because of the possibility of aortic dissection, or rupture. Escherichia coli is recognised as an uncommon cause of aortic mycotic aneurysm. An 81-year-old woman with a history of Escherichia coli pyelonephritis 4 months previously, was admitted to our centre for a mycotic aneurysm of the ascending aorta caused by Escherichia coli. She was successfully treated by urgent in situ replacement of the ascending aorta with a cryopreserved homograft, combined with antibiotics. Although infrequent, Escherichia coli mycotic aneurysm should be suspected in older patients with atherosclerosis and who developed septicaemia. Prompt treatment with a combination of appropriate antibiotics and surgery is required.