Seven hundred and twenty one abdominal aortic aneurysms were treated between 1960 and 1985. Twenty one of these (2.9%) were complicated by the development of a spontaneous primary fistula, 16 (2.2%) into the vena cava or iliac veins and 5 (0.7%) into the duodenum. A correct preoperative diagnosis was made in only four instances, two aorto-caval and two aorto-duodenal fistulae. Hospital mortality was 44% for aorto-caval and 60% for aorto-duodenal fistulae. Despite the lack of a precise preoperative diagnosis in the majority of cases, the prognosis for aorto-caval fistula remained comparable to that for patients undergoing emergency surgery for uncomplicated ruptured aortic aneurysms. The mortality of spontaneous aorto-duodenal fistulae was appreciably higher and the aneurysmal contents of 4 out of these 5 cases had positive bacterial cultures.