A patient with osteomyelitis of the lower thoracic spine is reported in whom computed tomography (CT) revealed a large false aneurysm of the descending aorta. Useful radiological signs of the diagnosis are described, namely a thin aneurysm wall, a multilobulated margin and deformity of the aneurysm by adjacent vascular and osseous structures. Mycotic aneurysms of the aorta have a high mortality and the CT demonstration of a pseudoaneurysm has a vital role in management.