Although the reported incidence of intraabdominal paraanastomotic aneurysms after abdominal aortic bypass grafting ranges from 1% to 15%, the true incidence is unknown because few studies have used routine, serial radiographic or sonographic imaging studies. Since July 1, 1988, we have used yearly abdominal sonography examinations to monitor our patients with aortic grafts. In the first 33 months we studied 138 patients. Medical records of 111 of these were available for review and form the basis of this report. Eleven patients (10%) were found to have intraabdominal paraanastomotic aneurysms ranging in overall size from 4.1 to 6.2 cm (mean, 5.0 ± 0.7 cm). The mean time between operation and detection of an aneurysm was 144 ± 101 months (range, 8 to 336 months). Three paraanastomotic aneurysms occurred within 3 years of operation, and the remaining eight occurred late (7 to 28 years). By life-table analysis, the incidence of paraanastomotic aneurysms was 27% at 15 years. Paraanastomotic aneurysms were classified as either pseudoaneurysms (presumed disruption of the anastomotic suture line, n = 7) or as true aneurysms (widening of the adjacent aorta, n = 4). True aneurysms occurred only after repair of an abdominal aortic aneurysm, whereas pseudoaneurysms were more frequent after bypass for occlusive disease. The finding of paraanastomotic aneurysms in 10% of our patients supports the use of yearly sonography for routine follow-up after aortic grafting.