Bacterial inflammation of the wall of the abdominal aorta leading to rupture or the formation of a "mycotic" pseudoaneurysm, has been described previ ously. An increasing number of such cases has been reported in connection with salmonella sepsis. A mycotic aneurysm or an aortic rupture has been observed together with lumbar osteomyelitis in a number of cases. In general, it has been assumed that the osteomyelitis developed secondarily, due to erosion and spreading of the infection from the aneurysm to the body of the lumbar verte bra, frequently L3. A case of erosive bacterial aortitis complicating osteomyeli tis in a 72 year-old woman suffering from diabetes mellitus and acute pyelonephritis is presented. She had a rupture of an otherwise normal abdomi nal aorta and simultaneous osteomyelitis. The pathological manifestation pre sumably occurred in the above order in this patient. Further, it throws some doubt as to the validity of earlier theories, as the lumbar osteomyelitis in some of these cases may have occurred primarily despite the fact that an aneurysmal dilatation of the aorta was present in the majority of cases.