An 82-year-old woman, who underwent axillo-bifemoral bypass for infrarenal aortic occlusion and peripheral arterial occlusive disease 9 years before, was admitted to our hospital for swelling in the left subclavicular region. Ultrasound examination revealed a leak in the wall of the bypass graft with the formation of a false aneurysm. No signs of infection, either locally or systemically, were observed. Resection of the aneurismal segment with interposition using a Dacron graft was performed. Macroscopic findings during surgery confirmed an intact anastomotic region of the left axillary artery and Dacron graft. Two possible mechanisms for the formation of this false aneurysm, either cumulative stress on the graft over the years or Dacron graft biodegradation, were hypothesized.